Rattled by Hum

Dear all,

Not sure if Home Theatre is right place for this - mod please move if not.

Would appreciate any thoughts on how to solve a recent hum problem with my AV setup.

I've recently plugged my PVR into the communal satellite feed that serves all flats in my block - but this has resulted in a nasty hum on the AV Unity Gain input on my SN1.  AV always used to hum a bit, but now it obliterates speech on normal TV viewing.  OK for Eastenders, but less good for Jools H.

Signal flow is:

Satellite wall socket > coax (x2) to Humax PVR > HDMI to Marantz 6006 AV amp > Front Left & Right Pre Out phono to Supernait AV (phono in) > NACA5 > Linn Keilidhs.

There's no hum on the centre & rear channels which are powered by the 6006.

However, I tried attaching the rear channels Pre Out to SN - they start humming!

I've changed power leads to SN and AV amp, and swapped phono leads between SN and AV amp - still hums.

I tried plugging the AV amp into the Tuner input on SN - still hums.

The communal coax box is behind my equipment rack.  I think this might be the problem...

If I unplug the coax cables, the hum stops.  So, I thought the hum must be in the signal path coming from the satellite feed.  Wrong (I think).  I tried unplugging the HDMI lead (so the path is broken) IT HUMS!  Plug HDMI back in - still hums.  Unplug the phonos (path broken again) IT DOESN'T HUM.  Eh?!?

Main rack (close to coax wall socket) is where the SN1 lives.  The AV amp is about 5ft away by the telly.  Hence the coax going to the Humax are c 6ft, and the phono lead coming back to the SN is also about 6 ft.  

I can only think that the feed from the satellite wall box is inducing hum in the AV amp (though this is 5 ft away) or the phono lead coming out of the AV amp.  Is this possible?  Is it fixable?

Any thoughts would be very much appreciated.

Cheers

Mark