i just recently i noticed a kind of popping/cracking noise coming from my sub. i'm running my Velodyne Deco 7.1 speakers system with an Onkyo TX-NR901 receiver. the crossover is set at 'deco setting' and the subwoofer level at a little less than half. when i watch movies, i have the master volume at around 60 - 70% of the maximum level. any help or suggestions would be greatly appreciated. please and thank you very much.