TDC is a moving target

[short-edd]

Hello all, I require knowledge;
As I turn the crank (anti-clock) My IT mark gets to within 20mm of the casing mark to line up TDC........and the fly wheel shoots past the casing mark by around a further 25mm.
Iv slowly (reverse) cranked it back in line and securely clamped the socket bar so nothing moves.
Carefully reset the valve gaps and it now sounds terrible (like a lollipop stick in a bicycle wheel but louder).
I have marked-out when the piston is at the top of the compression stroke but this bears little relationship to the timing marks
p.s. The bike did run well until I tried to stop the light tapping noise.
Any ideas ?
Edd