Noisy MH bulbs

[skewch]

Having a heck of a time with this, and am running out of things to try.

The fixture is a Sfiligoi XR6 with 3 x 400W MH's, each driven by an ACLS. The problem I am having is with the MH bulbs making an excessive amount of noise. Without a doubt, it is the bulbs, not the ballasts. It's an annoying high pitched 'hum'. I've tried 3 different bulbs:

Reeflux 12K - about 8 months use - no hum.
Sfiligoi 14K (came with the fixture) - hum.
XM 20K - extremely annoying hum.

And as you can assume, it's the XM 20K's I want to run in it.

I've tried isolating the ACLS/ballasts on separate electrical circuits, only powering one on at a time (in case of interference), still the same noise.

Any suggestions?