I had the timing belt replaced on my wife’s 06 Sienna last June. The serpentine and PS belts were also replaced. The serpentine belt has a tensioner pulley, but the PS belt is tightened by hand. On the way back from the shop the PS belt started screeching loudly.
I took it back to the shop to have it checked. The shop explained they prefer not to over-tighten the PS belt on installation. The belt tension was readjusted and there have been no further issues to date. Your shop’s explanation sounds reasonable.
PS = Power Steering, on the Sienna the noise would stop once the van had been driven for a few minutes. My guess is the belt warms up and had better grip. The noise did not reoccur after the belt was
re-tightened.
Personally, I’m happy the mechanic erred on the side of caution with the belt tension. Returning to the shop was only a minor inconvenience.
Ed B.