Product review (5 stars): OEM Whirlpool part
My old dryer was starting to rattle/groan when first started, and then the sound would go away.  Turns out if was a bad idler pulley.  Most sources say they whine or squeal when they go bad, but not this time.  Anyway, it was a 3 minute fix for less than $10, and this good old dryer just keeps chugging along.  Just pop off the front panel, pull the belt off the pulley, swap out the pulley, and put it back together.  I was able to diagnose this part as the problem by running the dryer and putting my hand on the pulley arm. This changed the pitch of the sound.