Maybe a valve not closing. The ignition is off, the cylinder compresses but the intake valve is not fully closed allowing it to blow back through the intake manifold like that. I would try a compression test and see how you get on. i would also check the timing is on point

Disclaimer- I do not know what I am talking about and on average it takes me about 6 months to figure out and fix what is wrong with my vehicles

1988 B250 Conversion Van