rattle noise on the left side of my 89 chevy cheyenne
 

Recently I have noticed a disturbing noise generated from the left side of my 1989 Chevy Cheyenne. It's loud enough to hear over my music and vibrate the driver door. (I don't listen to music now so I can focus on the noise maybe I'll be able to pin point it better soon)

This noise mainly happens when I am in drive with the break applied... if I let off the break it seems to get a bit quieter or dissipate. Also it doesn't seem to occur until the truck is warmed up say approx 10 min after start up.

I figured it may be old oil in the differential yet even though I bought my gasket ect the bolts are too stripped to do a full oil change right away (till I can get new bolts) so I manually siphoned the old oil and added new. The issue still occurs. There were some metal flakes in the oil, hopefully just from normal wear and tear. The next thing I'm figuring is the tranny oil. I know its important yet I can only afford one thing at a time.

I was wondering if anyone has had a similar issue and can let me know what fixed it?

I love my truck and have done a number of little repairs as preventative maintenance and aftercare yet this seems to be a serious noise that makes me worry.

So as I have been listening to the noise under my truck I'm starting to think it could be in the tranny :( the noise has come from under my feet (near the tranny) at some points in time. It almost seems that the vibrations are occurring in the center of my truck from under my feet through to the back. She is a very powerful truck 350 with 5.7 liter. I'm starting to think she may just be at too high of an idle. and the power is to much that she vibrates and then misc parts rattle. The Haynes tech book tells me that this idle cannot be adjusted?