tech advice required

[scooby222]

okay now this is a strange one but its really bugging me so any ideas gratefully recieved

went out to the car one night and when it started it didnt sound right,the exhaust sounds about half volume and theres banging and popping when i rev then lift off at both high and low revs. i had on a tsl firestorm backbox and ive put on a hayward and scott i borrowed but still quiet. like i say it seems to be running ok(maybe less resistance to the accelerator,but that could be my imagination) and ive had it checked on the diagnostics monitor and theres no fault codes, so any ideas?

cars an 05 sti uk ppp with green panel filter, 10,000 miles and was serviced about 1400 miles ago

[st3ph3n]

Tried running with the standard panel filter back in place?

[scooby222]

yep, no difference

[westtra]

Would banging and popping not mean over fueling? Could you have a boost leak causing this? or dodgy MAF making the car think its putting more air into car than it is?