You could be right, but FWIW...I don't have the Harmon Kardon, but aftermarket Alpine H/U with Alpine Type-R splits replacing whatever the factory junk was.
Sometime after the installation the system developed a buzz from the driver's side split, which like yours rose & fell with rpm. Thing was, some days it wouldn't do it, or at least not straight away...Did my freakin' head in.
I tried all sorts of things, took the panel off checked the connections - all ok, checked the harness to cone in the door, all good.
Eventually went back to installer. They were also bemused and after a bit trial and error, out came the head unit. In the end, as you know there's not a lot of room behind the head unit in the D2 and where the connectors from the head unit join the Disco harness one connector was getting pressed up against a piece of dash frame causing the interference.
So if you haven't already, check & double or triple wrap the connectors from the H/U...just in case. So simple, so frustrating - good luck! 
Cheers
Mike
'00 D2 Td5 'Alice'
'03 V6 Freelander 'Phoebe'
'04 Td4 Freelander 'Harry'
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