2006 buzzing from behind cab on acceleration

Especially when cold but most of the time anyway, my 06 Tacoma has what sounds like an electrical buzzing on acceleration and it appears to be coming from behind the cab. Sometimes it stops and starts again while I'm continuously applying power. Been doing this for several months now. Fuel pump?

 

A bit of a reach, but, depending on how clearly electrical your sound is:

On my '07, the top heat-shield on the catalytic convert ... detached from the base (it's just spot-welded on). This chunk of metal sitting on top the cat would rattle in a way that was *very* reminiscent of, though not the same as, an electrical buzz. It did not sound much like metal rattling against metal. It was worse when cold, and only made a noise (when it did) on acceleration. The cat is more or less under the driver, and the sound sounded like it came from under and behind the driver (LHD).

The noise drove me wonky (short drive) trying to find it. I only stumbled on it when I accidentally knocked my hand against the cat while under there one day. It clanked.