I use external speakers on my Macbook Pro, and lately whenever the furnace kicks on there's popping/crackling that happens in the speakers. It doesn't do it when I plug my Macbook (plastic) into the speakers. Could the metal of my MBP be picking up some sort of interference? i did a PRAM reset and a disk repair on my MBP, and it seems to have lessened it a bit. Anyone else ever had this happen?