SUDDEN KNOCK when a/c is on

yo fellas its like this recently when i was in a line for gas i felt the car knock or jilted a little so i immediately looked into the rear view mirror & noticed a car right behind me .I thought this asshole hit me from behind & i flied out te get on mad but he was at least 15" from me.I went back into the car a/c still on & still in the line car ideling & i got it again so i turned a/c of & it the knock dissappeared has this happen withany one i thought that it was plugs cause my plugs were badly off . I changed them today so i`mstill waiting to feel the noise or knock again :roll:

since then the car is running much better :twisted:

Could be an AC compressor problem! Take it too the dealer if you still have warranty!