Ground Loop? Noise from 12v to 5v converter manifest in bluetooth audio source
From your description it's probably a ground loop.

You're presumably supplying ground to your DC-DC and BT at both power in and signal out (to the amp).

I'd break the loop, probably by having the connection to the amp grounded at only one end of the cable.  I wouldn't break the loop at the power side because if you were to do that then DC-DC would have to use the amp's ground as a return path, which would be less than ideal.

An audio isolation transformer, like this one from ebay, should do what you need with minimal fuss.