The "hammeriing" ususally comes from the bottom of the shock's bushing. Wears out big time and gets worse with time. At above idel noise goes away.

Just replaced my this morning. Almost a no brainer on my 86 SDL. Major difference in noise level. Can finally hear the engine again.

Remove bolt and metal clip from top of shock. Slide shock to the left.
Remove nut and slid back bolt from top of tensioner spring.
Lift out tensioner spring. Hooks to the right side of the tensioner.
Tensioner will now piviot up on the left side to just allow the bottom bolt of shock to by-pass the fan pully. No need to pull fan pully.
Remove old shock and replace with new shock.
Reverse to re-install.

Check belt and make sure its riding on all pullys. check all bolts. Double check everything again.

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1986 SDL
My Other Car is a Porsche. When it stops leaking, I'm out of oil.