Indeed it is. Not sure where it might be getting some air from as the system is pretty much new front to back and I've gone over all the joints etc and can't see, smell or hear any leaks.
Unless it something to do with the smog system air pump permitting air in that way?
Might be worth checking
1. Control valve off smog pump
2. Smog checkvalves
These valves allow the air to go only from the pump and out the exhaust, so they are 1 way valves
Common for exhaust carbon to jam these stuck open so now air can not only enter exhaust flow, all the time
but also cause exhaust to flow backwards towards the pump
Easy to test by taking them off and can blow freely from the pump side
but cannot blow through from the exhaust side back to pump
If stuck, could try and soak them in engine cleaner or maybe gas
Else must replace or say F it and if no smog testing then rid that crap off or seal off.
Of course could be any air or exhaust leaks like at exhaust manifold gaskets
