This morning my R53 had a new sound under the hood that faded out gradually, but came back later in the commute. Sounded almost like the bearing in the tensioner is starting to fail, but who knows.

Is this a common failure mode for that part? The tensioner is original, and has ALWAYS had at least a 15% pulley on it.

 

Does it sound like a Hamster wheel squeek ? if so .... ."YEP..its fairly common" ...a good way to test if its in fact the tensioner get a spray bottle with adjustable nozzel and fill with water set to stream and spray tensioner wheel while engine is running and squeek sound is present...if it goes away ...thats the problem part...if it dosent spray the hub that comes out of your alternator that the belt is around ...And see if it stops then , sometimes the bearings will fail in those also.....hope this helps...used this process to find out where my squeek noise was.....It was the tensioner...