Sounds like the exhaust valve seal or body has been damaged?
Not an expensive item to replace - but a sod to change properly if you...
a/don't know how to strip, re-build & set-up a cocker
b/dont have the proper valve tool
c/are a bit of a numpty with a spanner & allen keys
Either way it's safer to get a good tech to sort it unless you're 100% confident