Hi all.
I had a really bizarre problem and thought I would share.
For a little while now I had been hearing a "bearing grinding noise" coming from my V6 1MZ-FE. I had the idlers bearings replaced at 93k because they were bad, and I thought that the upper bearing failed again. The timing belt was replaced at 60k and then later at 78k when I had to get the water pump replaced.
I took the car to my excellent Toyota mechanic and he pulled the timing belt cover and we had a listen. Much to our surprise, the noise wasn't coming from the idler but from the cam sprocket area (the cam closest to the front bumper).
After listening to the noise for a little while, we killed the engine and he thought for a little while...he checked the tensioner and it seemed OK and the idler bearings were OK too.
He then used a wrench to manually move the cam sprocket and we found the belt had stretched tremendously, almost by a tooth! The slack in the belt was beating against the upper cam and vibrating it which was actually causing the awful noise!
To make a long story short, he replaced the timing belt and tensioner, and the horrible noise went away and the engine is running very smoothly! I can tell a tremendous difference in performance and fuel economy has increased -- I suspect the noise was causing the knock sensors to falsely retard ignition.
My only residual concern is when we pulled the valve cover, I noticed some varnishing near the spark plug area closest to the cam sprocket, though I've been doing 6k oil changes with Mobil-1 5W-30. It's possible that the stretched timing belt somehow relates to the build up as I distinctly recall seeing the valve cover off at 90k mi w/o any varnishing...
Anyway, the bottom line is that if you happen to be in the timing belt area, replace the damn thing. A stretched belt can cause weird and hard to diagnose problems.
Jay.