Thunking Noise after Turning the Car off

[nathantaylor200]

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, Pressure in motor - knocks when I shut it off
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.

[34_40]

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??

[36 sedan]

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)?

[firebird77clone]

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.