I have a buzzing/droning noise from the exhaust when under load and in gear. Sort of like a "uhhhhh" sound. Noticed the engine is losing power-doesn't have the "oumph" it had, and I'm noticing a hesitation/ and or surge first thing in the morning, and on a cold start during the day till it warms up a little. recently installed Volvo plugs and aftermarket filter for fuel, but it really didn't change anything. No codes, OBD is clear. not really a big surge, sort of like I let off the gas, then back on, them off, then on. You can watch the tach move a little when it does it. Under idle, it's fine. The droning disappears if I let off a little, and it seems to happen most around 2000-RPM or more. Any ideas?