Yes, it is possible to experience engine knock after using seafoam. Engine knock occurs when fuel and air prematurely ignite in the combustion chamber. This can be caused by a variety of factors, including low-octane fuel, worn spark plugs, or carbon build-up. Seafoam is an aftermarket chemical additive used to clean out the carbon buildup in engines. It can help restore performance and reduce emissions, but it can't remove knocking if it is caused by anything other than carbon deposits.