Ill tell ya...I had an old Chevy Cavalier with the ol' 2.2 4 banger..& it had a knock like that, but I just drove it normally & never had a problem with it. Think it was a rod bearing that was making the noise, but it drove fine, never burnt any oil, etc. If I were you, I'd just drive it & deal with the noise frankly. Not gonna be a cheap repair any way you look at it, plus its going to cost you in inconvenience if nothing else. Keep it serviced like you're supposed to, & if the noise begins to be a regular tap/knock at all RPMs, then plan on doing something like what I described above. If you feel about your GTP the way I do about mine, you'd miss driving her!!