clicking noise on shut down...td5???

I turned the car off today and I heard a clicking noise on the passenger side. It sounded as though it was the air compressor or something as after it clicked it let out air I think? I heard this a couple of times lately particularly when I was having trouble with a flat battery and trying to turn it over. Am I right in thinking the air compressor or some unit like that is located on th epassenger side half way down the car? Any ideas of an impending problem (again:doh??