Lower engine air leak

I recently had a shop change out my A/C compressor on my 96 Contour (4cyl DHOC). (With EPA implications not a job I wanted to tackle, let alone the position of the compressor.) After picking up the car I heard a new noise and figured it was probably being masked by the A/C noise. I have eliminated the water pump and most other causes I can think of. The kicker is that the noise and a small oil leak coming from behind the A/C compressor mounts and I can see a bolt lying between the engine and the mount (no idea how it got there).

The noise sounds like a air being forced out of the engine. What I'm thinking is the mechanic loosened a bolt and dropped it when he was changing out the compressor, but as its been a couple of weeks I wanted some confirmation of my suspicions. I've never been to that part of the engine before. Is there a "port" into the lower part of the engine that is above the oil level and just behind the A/C compressor?

Thanks for any help.