Help! Rattling/buzzing noise at rear of head!
 

I'm having a bit of trouble since upgrading to an Ashdown MAG 410 cab last week. With it connected to a MAG 300W head there is a rattling/buzzing noise around by the rear of the head around the fan area, this only happens when I play the notes E G on the E and A strings.

Have checked all screws and opened up the head there's nothing loose or rattling, it sounds a bit like an electric buzz. This is strange as my previous 210 Ashdown cab works fine. I have checked all the ohms and stuff and the head and cab paired together should go fine as it's an 8ohm 410 cab paired with a 300w head with a minimum load of 4ohms.

Could anyone give me any advice? I've got a gig next Friday so wanna make sure i'm not damaging the amp in anyway.