what is the knocking noise after I have driven maybe 20 minutes? (non-consistant

What is your question?

it doesn't knock all the time, but after driving thru town i went to park and noticed a knock knock pause, knock knock knock, pause. car was idle in drive and park. also noticed some kind of knocking like it was coming from tire? just bought this car yesterday, and now I'm worried. the elderly man i bought it from thought it was the same noise from the air conditioner.