Did a shop do the repair for you ?? If you paid for new shock and new tensioner then I would go back and insist that they re-work it and make it right. When correct this belt is "silent".

A think new shock would normally come with new bushings in it. Call Phil at partsshop and confirm that this is so. The sound you describe is a lot like I remember when I had a top bushing with some play in it. While idling carefully push (with pressure of 5 lbs or so) on the top bushing using a long handled tool --- If you can feel any movement or if the sound changes you can tell if the bushing is worn. I'm suspicous that they may have re-used parts. Is that a possiblity? If they re-used the tensioner it may be set up too tight. I shared my experience about re-using and setting tension on the tensioner in another post recently. Do a search for DanielW posts and it will come up.

Another possible is that an idler pully has a bad bearing and is about to go out, or a fan clutch bearing. Is there any noise from twirling the fan ?(With engine OFF, of course)

Good luck,

DanielW 89 300E