Weird noise... "BUK BUK BUK"..."BUK BUK"

[RustX7]

This noise only occurs every so often, and I think it may come from the rear but I'm not sure. I feel like I've heard it from more than one location. It's a fairly low pitched knocking "BUK BUK BUK" sound that always occurs in groups of 2, 3, or 4. Most often groups of 3. Usually at very low speeds, like as I'm coming to a stop or shortly after I've begun moving again. The BUKs come at a rate of about 2 per second. They almost sound like someone hammering in a small nail or something. It's nothing even remotely irregular like something clanging in the trunk or hanging from the undercarriage. It's more like a pump or something that's trying to build up pressure or something. When it does happen it will only happen once or twice per trip.

Any ideas? Could this be a transmission noise?

Forgot to mention it's a 2001 LeSabre.