Yesterday my truck started making a low growl/whine sound at low rpms. I could not hear it at driving speed but when I slowed down and the engine dropped below 1200 or so rpm, the sound became apparent. It's not driveline because it follows engine rpm regardless of speed. I can't locate it. Any ideas? Thanks.

 

Well, take a yard stick, or a broom handle, and hold it to your ear and touch various things while at idle. Like alternator, AC compressor, idler pulleys, valve covers. You can eliminate things that are belt driven by removing the belt.