So, Just The Heat Shield

The source of the buzzing sound when I accelerate?  The heat shield covering the bottom side of my car's catalytic converter started to rust, and that apparently widened holes to the point where the shield was able to rattle from time to time.  He ripped it out, said it's better this way.  It's only there, he says, to prevent the converter from, say, lighting a pile of leaves on fire.  Knowing me, I'll park under a pile of leaves and set that and thus my car on fire.

So that's the thing that caused me so much anxiety and anguish the past couple months: A heat shield.  It had nothing to do with the performance of my car, so I theoretically could have driven my car with it still on, rattling as it would.

Everything else appears to be fine: After six months and about 5,000 miles I wanted the oil changed; after three years I really needed the brake fluid changed; and the tires were rotated after they were put on in April.  The mechanic took the initiative of cleaning the rear brakes, which tacked on some money, but I trust that it will help with the performance of the car, so I'll accept that, even if it costs me $300 total.

Don't have to worry about the car again, at least for a while.  Yeah, this is good.