Update: After noticing a rattling sound coming from inside the right half of the dash I decided it was probably
related to vibrations caused by the speaker. I installed an XTC slimline speaker baffle behind each of the front speakers
and also used cushion tape between the adaptor plates and the frame.
I've definately noticed clearer and louder treble thanks to the baffles, but most importantly the unwanted noise seems
to be gone.