Ewww...

Ok, try this...

Empty the tank, and then hold the plane nose up without the wing (someplace where you have good light). Then gently wiggle the nose and see if you can either see or hear the clunk moving around.

If you can see or hear it, the problem is elsewhere. (although it could also be that the fuel line has become disconnected inside the tank which would require removing the tank anyway). If you cannot see or hear the clunk, try the shaking method, sometimes it works, but not often. What the plane needs is a good rearward shot (similar to the forward hit it took when it hit the tree). Keep trying till you can see or hear the clunk.

As far as removing the tank goes, it would help to know what what type of plane it is.