Why do my plumbing pipes vibrate?

The pipes behind my shower vibrate. I don't think that I have "water hammer" because the vibration occurs at very random times both when water is running and also many hours later. The pipes do NOT rattle, knock or bang...but they vibrate (very loudly...enough to startle me in my sleep) and the sound lasts for 3-4 seconds. The sound seems to localized in the wall behind my shower. Is the water pressure a possible cause? I have lived here for 10 months, and the problem began last month. My plumbing is about 10 years old. Any ideas?