Engine Lob

Just replaced all the spark plugs and coils. had some replaced (about 4) earlier but the engine still seemed to be missing as in like a tune up of yesteryear. The car ran like a dream for about a week. Now it is sounding and running worse than when it was before the repair. I had the work done by a private mechanic friend. This work is not that complex. Now, we opened the hood and could hear a hissing sound like air being sucked in somewhere. We can't find it but pulled a few suction lines and it seemed to get better when they were covered up by a thumb. Still can't trace the potential air leak. Now I told my friend to make sure that the coil/plug covers were somewhat sealed as that is what we were told to do. Nothing fancy, but the information from Ford said that those covers should not leak any air. The mechanic said it was not that bid of a deal. Now, what can I do? How can this be tested with some reasonable budget and how can we eliminate the good suction lines without replacing all of them cart blanche? Are there other things we should test for too? Also, NO CHECK ENGINE LIGHTS ARE ON. NO CODES, JUST a care running like an old care out of timing.