I had the same 'Angry Bees' sound start emanating from under one of the 90's Old's (
still have a couple 'Oldsmo'bubbles'
in the family fleet - my dad L-O-V-E-D Oldsmobiles and I keep them in his honor
) and it turned out to be the failure of the 'Honey-Comb' element internally in the "Capitalistic Convertor
This was a "MagnaFlow"* replacement unit and they replaced it with no questions asked - it to was @2~3 years in service before the failure.
The car passed 'Smog' both times before the failure ...
I often scanned the ECM to see if there are any Pending codes during routine servicing and never saw a problem prior to the failure.
The car still runs fine with no other issues and this unit is now @3 years old ...
If your mechanic says there are no internal or fuel system issues contributing to the failure I would advise them of this - in writing - and would request they replace it (in writing) - and forward to CARB if they bawk or give you any problems ...
* = BTW - I buy these through 'Summit'
Most don't realize they sell these at GREAT
pricing compared to any other source I can find.
One little problem though - they can't/won't/will not ship to CA due to CARB regulations - so you have to go pick-up - just sayin' that's how I'd do it if I ever had to