Knocking sound at low rpm in 2/3/4 gear

[KJM1511]

I have a 2000 Mazda B2500 4cyl 5 speed. When I’m driving at lower speeds/rpms for the gear I’m in there is a pretty noticeable knocking/rattling sound coming from what seems like the front of the trans/back of the motor. Also get the noise when coasting in gear, and it goes away when the clutch is pressed. Can’t quite figure it out, hoping it’s not internal in the trans.

 

[Brain75]

Basic diagnostics, does it increase/decrease with mph (in the drivetrain from output shaft back)? does it increase/decrease with engine RPM? (in the engine) does it go away during braking, firm acceleration, coasting out of gear? does it only happen when in certain gears 2/3/4 but not any other - ever? does it do it in reverse?

 

[KJM1511]

Basic diagnostics, does it increase/decrease with mph (in the drivetrain from output shaft back)? does it increase/decrease with engine RPM? (in the engine) does it go away during braking, firm acceleration, coasting out of gear? does it only happen when in certain gears 2/3/4 but not any other - ever? does it do it in reverse?

It stays pretty constant as far as different speeds, does seem to change with rpm, for example if I’m in 4th gear and I get below like 2500rpm the noise is really noticeable unless I downshift or accelerate and get it up to 2700 or so.

 

[Brain75]

If it changes with RPM and not with mph, than it is most likely in the engine... could be the input shaft side of the transmission since that is directly tied to RPM too, but halfway through the trans you loose that association.
Given that, and the fact it goes away when the clutch is depressed (stopping rotation of everything in the front half of the trans if it is out of gear and clutch depressed), then it is almost certainly in the engine.

Given the year (2000), and the fact you didn't specify engine explicitly... I got to ask, was the B series a 2.5 Lima in those years as well? Didn't start the Duratec a couple years early or anything? I ask cause I just don't know the little oddball exceptions the B series sometimes had.

how many miles are we looking at, same on both engine and trans?

do your own oil changes and look for metal bits in oil?

No other symptoms other than that - no issues getting in gear, staying in gear, etc...?

 

[pentode]

Brainstorming here ... This sound only happens in 2-3-4 and not in 1st or 5th?
Unless I'm missing or overlooking something, I'm thinking if the knocking stops when the clutch is depressed, that rules out the engine. If the noise happens coasting in gear, but stops when the clutch is depressed, that rules out everything from the main shaft/counter shaft back. That would leave the pressure plate through to the input shaft and associated bits.
It's the only 2-3-4 gears part that has me scratching my head.

 

[KJM1511]

If it changes with RPM and not with mph, than it is most likely in the engine... could be the input shaft side of the transmission since that is directly tied to RPM too, but halfway through the trans you loose that association.
Given that, and the fact it goes away when the clutch is depressed (stopping rotation of everything in the front half of the trans if it is out of gear and clutch depressed), then it is almost certainly in the engine.

Given the year (2000), and the fact you didn't specify engine explicitly... I got to ask, was the B series a 2.5 Lima in those years as well? Didn't start the Duratec a couple years early or anything? I ask cause I just don't know the little oddball exceptions the B series sometimes had.

how many miles are we looking at, same on both engine and trans?

do your own oil changes and look for metal bits in oil?

No other symptoms other than that - no issues getting in gear, staying in gear, etc...?

Yes it’s a 2.5, 227k on the odometer, I’d imagine it’s the original transmission as well. I just bought the truck a couple weeks ago so I haven’t gotten into it much yet. I’ve been a mechanic for about 10 years so I do all my own work just haven’t had a chance to really go through this one yet. And no other symptoms either, it goes into and stays in gear just fine. I’m beginning to think the previous owner may have done the clutch/pressure plate shortly before I bought it because the slave cylinder is brand new. Driveability doesn’t seem affected the noise is just obnoxious, just trying to see if anyone has any B series secrets before I start going through the possible causes one at a time

 

[KJM1511]

Brainstorming here ... This sound only happens in 2-3-4 and not in 1st or 5th?
Unless I'm missing or overlooking something, I'm thinking if the knocking stops when the clutch is depressed, that rules out the engine. If the noise happens coasting in gear, but stops when the clutch is depressed, that rules out everything from the main shaft/counter shaft back. That would leave the pressure plate through to the input shaft and associated bits.
It's the only 2-3-4 gears part that has me scratching my head.

I’ll have to pay a little closer attention to 1st and 5th to be sure, it’s most definitely more noticeable in 2/3/4. More of a bad roller bearing sound than a knock I suppose, thought it was the throw out bearing at first but it doesn’t make any noise with the clutch depressed like a throw out bearing would. Once I get a free weekend I plan on pulling the trans and taking a look at the disc/pressure plate/flywheel etc.

 

[pentode]

Oh, if it's a bearing noise, I think it can really only be the input shaft bearing. Shouldn't be the worst job in the world to replace if that's the case.

 

[KJM1511]

Oh, if it's a bearing noise, I think it can really only be the input shaft bearing. Shouldn't be the worst job in the world to replace if that's the case.

Just found the owners manual stashed in the rear fold up half seats, it has an interesting little piece of information about this exact issue. Definitely puts me at ease a little, but I’ve been driving stick for 20 years, and I know I wasn’t shifting at “extremely low” rpm’s. Still gonna look into it but apparently it came from the factory this way thanks everyone for your input

 

[pentode]

Nice find - now my question is what is "extremely low rpms"? That sounds like lugging the engine, which is something you'd definitely want to avoid.

 

[KJM1511]

Nice find - now my question is what is "extremely low rpms"? That sounds like lugging the engine, which is something you'd definitely want to avoid.

I’d imagine that’s what the manual is referring to but I don’t do that to any extent that would be considered lugging. I usually downshift around 24-2500 rpm’s depending on how quick I need to slow down or accelerate back up. The manual shows their recommended shift points but it’s all in mph not rpm, 1-2 at 10mph, 2-3 at 22mph, 3-4 at 33mph and 4-5 at 41mph. To me those speeds all seem way low to upshift in this truck, and I always shift my rpm anyway, almost always right at 3000 or above if I’m climbing a hill or want to get moving

 

[pentode]

If you're normally driving in the 2500+ range, you're definitely not lugging it. . But then I don't think what the manual is talking about applies to you either.
I wouldn't expect any engine to lug running over 1500rpm other than under serious load.