Thunking Noise after Turning the Car off

Disclosure: I am new to car mechanics so please be patient with me.

What I have:
I have a 1965 Impala and just rebuilt a '74 350 for it(cast-iron), please spare the criticism for not sticking with the 283. The new 350 also has vortec heads and I also installed a flex fan.

The Issue:
About a third of the time when I turn the car off I hear this thunking noise that really sounds like the flex fan is hitting something because it slows down like, "thunk..thunk....thunk........thunk" but the engine definitely stops right when i turn the ignition off so it is not a run on or anything. I have also verified that the fan is for sure not hitting anything even though that is how I described the sound I am pretty sure that is not it. I did find this forum, http://www.clubhotrod.com/shop-talk/...-shut-off.html
that mentions a possible pressure release happening but no resolution was posted. Overall the engine runs great, a little bit of an oil burner but nothing to complain about. Any help is appreciated.

Welcome Aboard, Nice to see you here.
Have you done any checks yet? I'd be checking things like the flywheel / flexplate, torque converter fasteners ( if auto trans car) or perhaps pull a fan belt off and isolate the alternator or power steering pump (if equipped) or water pump??

I had a vacuum brake booster with a bad check valve that made some crazy noises when it was shut off, lasted about 30 seconds or so (if memory serves)?

Sounds like you have taken some flak for upgrading the engine.


Don't let it bother you, it's a good upgrade.

You might try having someone shut down the motor while you are under the hood listening.