a knock (deep) or a tap (higher pitch)?

if it is more of a tap it may just be the valve adjuster, which is not to worry about and often can be solved by a change in oil type.

a knock can be serious.

main bearing noise is not a good thing.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.