Knocks knock

Naturally aspirated tech and talk
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Belgian951
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Hearing something like knocking after standing still for another few months.

My thinking is:
- old gas with too little (and too late) added stabilizer is bad
- vacuum leak (hear hiss)

Results in the knock.

Is that thinking correct or would these two factors not contribute to a knock?
1986 944 Turbo Garnet Rot Metallic

#1