This is a follow-up response to my earlier posting titled "Backfiring without jerking".

I follow Tucker's instructions and went back to Volvo workshop to have the car checked once more. They are suggesting that it could be the exhaust pipe. I am lost here because the exhaust pipe, front, mid and rear end were changed only recently.

Those engine valves that were changed are those inside the engine and there are five of them. I don't know what valves are those. Accordingly the camshaft is in good order and volvo mechanic told me there is no reason for the flywheel to cause backfiring.

Humming Sound

As to the humming sound at 1500 - 2000rpm, I failed to mention that the sound comes only after about 10 minutes on the road. When the engine or whatever part is cool, everything is nice and quiet. Volvo mechanic told me that my car has only 1 fuel pump in the petrol tank and they assured me that it is not the fuel pump that is causing the humming sound since acceleration is okay. What else to check and do?

Thanks