How can a police officer pull you over for loud exhaust?

i was coming home at approx. 1215 AM and i turned on a road where theres a park and ride at the corner..a cop just happened to be sitting there. he had his lights on and was investigating another vehicle sitting there. as i turned on the road i saw the cop heading back to his cruiser...i made it another 1/2 mile and saw headlights pull out of the park and ride and heading my way..i was almost to my buddies house, but i missed the driveway( it was dark, raining and in the middle of the night) so i pulled off in the next driveway but did not pull all the way in, just far enough so i could turn around..well then im sitting there waiting for the car that was coming to pass...but it happened to be the cop, and as i just sat there, he pulls over and turns his blues on...he comes to the car and does his thing..in the process he asked me if i had an exhaust leak..i said no and that less then 8 hrs prior to this my car was in the shop taking an inspection sticker...i needed exhaust work to take a sticker...i got it fixed and got the sticker...anyway that didnt seem to phase him...he looked by my shifter and saw a roach...he went back to his car for a few moments and returned...he said he needed to do a vehicle search, so he did( as far as i knew, two cops should be present during a vehicle search..well there was only one. but he was a sheriff so i dont know if that makes a difference) anyway he searched and found my stuff.. he then takes the time to write the summon's up, at that time the second sheriff pulled up. he came back had me sign them.. then he has me do a soberiety test...where he made me do one was on a hill, and the road had tons of uneven and cracked spots..after he says im going to be arrested b/c he was under the assumtion i was under the influence. my friend i had with me walked back to his house as i was heading to the police station. my main defense i think is the fact he pulled me over for my exhaust being to loud, but in fact wasnt to loud seeing how if it was, dont you think i wouldnt have been able to get my inspection done? so please read through this, let me know if anything the cop did was lawful/unlawful and if i may have a good chance when fighting this. thank you!!