Noise under rear driver seat???

06 Tacoma 4X4 - Need some help diagnosing a strange noise that seems to be coming from the fuel tank area. It started a few weeks ago and I can only hear it inside the cab of the truck while a low rpm's. While in park it sounds like a slow inconsistant tapping, when I shift into R or D (rpm's slow) the sound gets louder and sometimes more consistant. If I hold the break and increase to 1100 rpm's it gets a faster and more consistant. Past 1100 rpms the engine noise drowns it out. When I slow to a stop it comes back and at times is very loud. At the "stealership" they changed the manifold and converter, then changed the belt tensioner pully assembly and nothing about the noise changed. They told me to drive it and when it gets louder to come back. Sounds to me like it may be the fuel pump. Any advice???

Thanks