I had some repairs done on the car, and when I got it back there was a hiss type sound present at idle, but gets much louder when pressing gas pedal during acceleration. The hiss does not get louder when revving engine when parked. Did some googling and it seems to say it could be a vacuum leak. I listened to where it is coming from, and it seems to be around cylinder 1 and 3 area, maybe tubes around there? I tried to listen if its coming form the belt area, but I don't think it is. Seems like its on top of the engine, maybe between engine and firewall on the passenger side. Whats weird, is when driving, it seems to be coming from driver side. Also, sometimes when at a red light, there is a few momentary engine rattles as if there is a misfire in one of the cylinders, off and on, and only on some days, today there was no rattle at all. I've had a faulty coil cause that issue before. Dunno if its another coil again, or maybe the rattle could be related to the hiss. Check engine light is off, I read the blinking codes and it said only 0304 (knock sensor), had that code for ages.

Also, my belt squeaks a bit when turning car on sometimes. And it also squeaks when turning on AC soon after starting the car... after a few mins of driving, turning on the AC does not cause the squeaking. I will probably need to replace the belt soon.

I'd greatly appreciate any input.

 

Vacuum leaks - misfiring ignition coil/bad spark / clogged injector/ bad fuel filter - bad maf