I have a Budget Y pipe bolted on to a 3.5 engine. Only when the car is in a forward moving gear, and only at idle I hear a marbles in a can, bees in a can type noise. It can be best described as the sound that an A/C would make from the portion thats facing out of a window when it's cooling a room. What can it be? And why does it only do that at idle in a forward moving gear?