OK, I sat down on a stool in front of the cab with the grill removed & had a right good listen. The 'flutter' was coming from the piezo unit. I disconnected it & screwed it back on, but the sound was still there - coming *through* the slots in it - it's not a sealed unit!. I removed it leaving a 75mm diameter hole. No flutter. Any suggestions for a sealed replacement would be welcome, or, alternatively, the best way to seal up the hole that's left.
G.