Loud humming after using A/C for a while

[4.0blue98]

I've done a search here and didn't find any related threads so thought I'd start a new one. I've got a 98 4.0 OHV that develops a loud hum after using the A/C (reg or max) for about 20-30 minutes. A/C works great all the time. Compressor cycles like always and no belt squeal (new belt too). Immediately cold and blows cold even while making tons of noise. It continues after shutting off the A/C and also after shutting off and restarting the engine after a few minutes. If you rev it will fade but come back when idle resumes. If I don't use the A/C no noise. If you shut the engine off and wait a few hours no noise at restart. Haven't tried shorter times. I've heard a bad or dirty IAC can cause some issues like this. Don't think it's the IAC since the A/C is required to trigger the problem, but have not checked the IAC.

I'd have thought low charge but plenty of cold air. I'd think if the compressor were toast I'd hear it all the time. Any ideas?? It's hot out there!

 

[Earl43P]

You can disconnect the electrical connector to the IAC and see if the humming changes or stops.

 

[4.0blue98]

Thanks for the reply Earl43P.

I haven't been able to get it to make that noise since our outside temps dropped below 85. I've been using the AC as I had the 5 or 6 times it did. I will try to unplug the IAC if I can replicate. Just didn't want you to think I wasn't grateful for the reply. I'll send the thread an update if I can figure it out but seems like it isn't the compressor since everything is working right. I'm definitely leaning toward the IAC.

Thanks again, and thanks for your service to our country!