1998 Honda Civic strange knocking sound

I've recently noticed that my car is beginning to make a knocking noise when I am driving. I can sometimes feel it underneath my left footrest as I'm coasting slowly. At first I thought that the fan belt was acting up, but the performance in normal. Then I thought that maybe something was loose, but after I took it for a service and mentioned that it was knocking, they said nothing was out of the ordinary. Is this because my engine and parts are almost at the end? Or can I just tighten some bolts somewhere and be rid of it?