My receiver is a 2002 Panasonic SA-ZR45. For the past few years a crackling sound has penetrated the speakers. When I shut the receiver off the crackling gees away. Until recently. Now, the crackling is on all the time and I had to switch to the TV speakers, not too good. What could be the cause of he crackling, and how can it be fixed? Or, should I invest in a new receiver?