This happened last year but I'll share because I feel I learned a valuable lesson.

While the p/u ('68 GMC) was down for the winter I did a few projects. Started with lowering, 3" drop spindles and 4" coils. Front lower control arms needed some rework due to clearance issues with 15" tires. Took a while and it was still tight but not rubbing. Next I relocated the rear shocks to correct the angle (lowering flattens the angle to the point of almost horizontal) The kit also relocated the shocks to outside the frame rail. New adjustable and longer pan hard bar. Why stop there? I want this truck to handle. Moved the engine back a few inches. Next was the power steering upgrade. Removed the box and linkage and installed rack and pinion. This now interfered with the sway bar (still sorting that one) These trucks come with somewhere between 1-2 degrees caster and I'm hoping for about 5. So I removed the lower control arms and relocated the cross shaft forward about 1/2", more clearance issues at the rear of the control arm. Bit of grinding had that sorted. On test drive it handled beautifully, even without a front sway bar. But now there was a wretched grinding noise on low speed cornering. I took everything apart, checked and rechecked and even added a little more clearance. Still grinding. I finally realized the noise was coming from the back. With all the major changes and rework I completely forgot I had changed the cheap, chrome and leaking rear end cover. When I refilled the rear-end I didn't add friction modifier. I had used royal purple before and didn't need any. Now with amsoil I added a bottle did a few figure eights in each direction and the noise disappeared. 

Such a simple thing yet I spend days pulling my hair out checking and rechecking everything.