4.6 Vibration at 1100RPM

I have a 2002 P38 4.6 which has a resonance or vibration at 1100 RPM it starts at 1000PM reaches a maximum at 1100RPM then is gone by 1200RPM. It can be felt in neutral or park with no load on the engine but becomes a lot more noticeable when the engine is under load.
I thought it might be the Torque convertor but I have changed it and it is the same. I now think it might be the exhaust I have changed the centre box, just the same.
Has anybody else experienced this?
Also the obvious question can anybody please help with ideas?
Thank you

Missing on 1 or more cylinders, torsional vibration damper?

Can you see the engine shake a little if you rev it a little with the bonnet up. Try lifting the throttle cable up a tad (its the one furthest away from you - or should be). Could be plugs or leads, have you changed them lately. Did you do the torque converter or a garage? Was the flex plate changed at the same time and was the torque converter a new or re-con one or second hand??
Can I also ask how many miles has the car done.

Thanks for your replies.
I do not think it is missing on a cylinder but it is a good idea I will check it out.
Torsional vibration damper - Is that the crankshaft pulley? can they fail?
I cannot see the engine shake if I rev it up but I do need to get someone to do it while in drive and holding the brakes on, it is 10 times worse under load.
I did the torque converter it was second hand but only 60k, I also fitted a new flexi plate.
The car has done 47K but I have had it from 26K and it has always done it.
I may try the engine mounts it could be that the rubber has become hard.