TANNOY REVEAL 66D ACTIVE MONITORS CRACKLING?

[rumpelstiltskin]

Hi,just bought a pair of second hand Tannoy reveal 66d active monitors for home recording.Problem is they seem to crackle when you first switch them on?But after a minute or two this disappears and they are perfect?When you turn the trim knob at the rear of the monitors there is crackling too,anyone any ideas what could be wrong?Cheers.