engine knock

Hello every one.
I have 91 lesabre . It has a knock that occcurs after running car for a bit. It starts and warms up well.Idles excellent. I drive to work takes 20 minutes. when I arrive I must turn the car off for five ten minutes and do duties. I come back out ,start the car ,it idles smooth ,quiet and excellent again but then when i put it in gear for about 100 feet it will knock hard ,and then the knocking smoothes and fades and the car is fine.Never does this on the intial cold start and drive or at idle.. (This is a thunk thunk sound.right hand underside??.) ive checked balancer and it doesnt seem top have any play in it and the belt runs smoothly too. Could it be harmonic balancer( worn rubber) or ac pump. havent tested without belt yet. Any suggestions appreciated. In adavance ,Thanks for all your help--again.
Steve

Hi steve here again, ive checked motor over again.Cant find anything loose. What about a worn motor mount(s).Car has always been well oiled.Perhaps some rot there.How do you check those for wear.???
Thanks

There is no way to tell what this is from here, you or someone with a sharp ear for engine? noises must listen to this could be the balancer, could be the cam thrust button ( common issues both). Mptor mounts would clunk once on accleration as a rule.

Thanks Maxwedge for the suggestion. Hmm , yes, Balancer seems to have no play in it when I lay hands on it but cam thrust button. Where is that? Didnt know there was a button.When it wears out does it stick or misalign,
or mis-shape? How easy is it to replace? Im learning.
Steve

This is under the timing chain cover it is a spring loaded " button' that prevents the cam from moving in and out of if it's bore. Usually rattles or knocks at unloaded steady engine rpms. This can be tough to pinpoint.

if it only knocks when in gear and moving then it's in the driveline somewhere

if you got the 3.8 or 3800 in that they are bad on harmontic and it's a real bad noise like the whole moter knocking. if you take belt off and turn harmotic back and forth you should hear it knock just by turning it back and forth and if you do this and theres no play in it it's not harmotic balancer then you could rual that out for sure.

The rubber breaks down on the balancers and this is a common problem

Agreed on the balancer, but it usually makes noise even sitting still, none of us can really pinpoint this from here.

it idles smooth ,quiet and excellent again but then when i put it in gear for about 100 feet it will knock hard ,and then the knocking smoothes and fades and the car is fine. Sorry this part threw me off.