HT-5210 Effects return noise

[Idlefrog]

Hi,

I have recently bought a Line 6 Firehawk FX unit and plugged it into the FX return on the HT-5210. I started to hear pops and crackles from the amp. After some experimenting I have found that whenever I plug a lead in to the effects return socket, the amp starts to pop and crackle even without the other end of the lead being plugged in to anything. I have tried several leads. As soon as I pull the jack out the popping stops. I found one thread similar which suggested dirt in the jack plug socket Does anyone have any similar experience to this or any advice?

Also, would there be any sound difference in using the MP3/Line in socket with the Firehawk rather than the effects return? I'm not currently interested in the 4 cable method, I just wanted bypass the preamp and basically use the amp as an active speaker. I get no pops or crackles using the line in.

Thanks for you help.
Andrew