Plumbing and Piping - Air lock?

Please help...
My water pipes are making a vibrating noise in the attic and walls. It comes and goes on a cycle of about 30 seconds, and is a low, rumblimg sound.

This happened a couple of years ago, and I was told that it was an "air lock", that some air had become trapped in the pipes and was causing the vibration. We drained the pipes, refilled them, and the problem disappeared.

This time draining the pipes hasn't seemed to work. I drained them yesterday, and the problem went away for about 24 hours. I heard the noise coming back, so I drained them again today. When I refilled the pipes, the noise started up again immediately, and it's louder than ever.

Anyone know if I'm missing a step?
Thanks...

Are you on a pump/well system or a public water system?

Do you know if you have water hammer arrestor's installed? They would look like this:


http://www.plumbingsupply.com/images/wh_arrester2.jpg

When you drained the system, did you open the faucets on all tubs, basins, and sinks?

Thanks for the replies. I'm on a public water system. I don't have the arrestors installed, and I did open all faucets and let them run until dry.

The problem seems to be corrected for the moment. After a couple of the faucets let out their last few air bubbles after being re-filled, the problem has abated. I'll keep my fingers crossed.

Thanks again...

If you're on a public water system, you may have a Pressure Reducing Valve that needs adjusting, repair or replacement.