squeeking noise

my 2003 Prius has started having a noise which I thought was maybe an exhaust leak due to the level of sound. I had parked on grass where I think the bottom touched. It is intermittent and I thought maybe it was from acceleration but it is not consistent, sometimes starts on a turn or when breaking. code said the 4th cylinder was not firing so I replaced all plugs and the 4th ignition coil...it seems a little quieter but I cant be sure. I ordered 3 more to put in asap.
Meanwhile I decided to take car in to get exhaust system checked and the guys there said no leak but they think it may be motor or other mounts but I should check to see if this is common before they would go ahead and replace these on motor. They also said if there was a cylinder issue the check engine light should be on and the diagnostic machines won't have anything to test if that is not on. Is this all true . Does it sound like a mount issue ? Is that common? my car has 210,000 miles on it now...regular oil changes but I never did a tune-up since the purchase at 150,000 miles...my bad the plugs looked like thin needles!!!! could this mess up a piston or cylinder?

They also said if there was a cylinder issue the check engine light should be on (which is not on) and the diagnostic machines won't have anything to test if that is not on. Is this all true . Does it sound like a mount issue ? My car has 210,000 miles on it now...regular oil changes but I never did a tune-up since the purchase at 150,000 miles...my bad! the plugs looked like thin needles!!!! could this mess up a piston or cylinder?

Any help greatly appreciated!