High Pitched Whistle

[norcalford]

1999 Ford Ranger 3.0L V6 4x4
138,000 miles

A few days ago I changed the oil and replaced the oil filter on my truck. (Used a Motorcraft oil filter and Castrol GTX Super Sludge.)
Since then, there's been a high pitched whistle when I travel at speed -- I'd say about 25-30 mph and above.
I never paid any attention to it, because it's an old truck, but all my friends who get in ask me about it. Being asked about over and over has put some anxiety in me about the noise.

Any ideas? Anyone else's Ranger make that noise?

 

[masterbrenden]

it is speed related not rpm related? you say 25 mph but what rpm is the truck at when it is making this noise?

 

[norcalford]

Yeah, sorry for the confusion. It's not related to the rpm ...so most likely not engine related?
The whistle only starts when I'm rolling at around 25 mph or faster, and when I slow down it goes away.