Loud intermittent noise on the promenade?

For the past 7 years I have had a problem with an intermittent LOUD vacuum sound. It almost sounds like a jet engine. I cannot find the exact source, but it seems to be coming from one of the big buildings on the water side of the BQE close to Pier 1 (3 blocks away from my apartment). It comes on for anywhere from 30 min's to 2 hours. Tuesday morning (5/7/07) It lasted for about 3 hours (from 8:00- 11:00) . This was the first time I was able to find the approximate source because it's so intermittent. It happens at night as well. It is not just a neighbors vacuum. It's something really big. Maybe a huge generator powered by a turbine? Has anyone else heard it. Am I crazy????