Sounds almost like an exhaust gasket leak.
Sorry to sound abit silly here but when you say gasket , do you mean the graphite ring where the two pipes joint or at the front where it meant the engine ?
Where it joins to the engine.
I will check this tomorrow and report back , as I’m going to recheck the valve clearance for my own piece of mind aswell as it just sounds very tappy but I’m certain it’s all tight and the gasket was new.