Is the thrum thrum thrum happening when no source is connected? If not, it could be the source.
I have a set of DSP5000s (24 bit) and one of them squeals when powered on like yours. I asked Chris the Meridian repair guy when he was repairing stuff about that and he said don't worry about it... some speakers do that! I guess he didn't find the cause.
My workaround is to quickly flick the switch on the rear of squeaking speakers very quickly on and off... then on again. If I catch it right, it doesn't squeak.
I bought these speakers 2nd hand non working. I have had to play around with them to get them working and from memory I think the 'problem' is on the Amp board as I have swopped them over and the squeal moves with the Amp board. Other than that, they work OK and sound really good.
Main: 561M, DSP5000 x 4, DSP5000C, 598, 504, 2 x M&K850 Subs, Sony Blueray, LG 50" Plasma,
Attic: 565z3, 500, 562V, Audiolab 8000 C+P, Rega Planar 3, Michell Mycro, various cartridges, various tonearms, RPM 5.2, Musical Fidelity X-LPS V3, Rogers LS55