yip, gotta be a leak somewhere, try to isolate it, could be that the pipe isn't mounted 100% flat, a pinhole leak in the exhaust pipe, etc, and note: i had a similar problem on a 787 where it wouldn't smoke inside the hull when on the trailer, but only when the exhaust was under water (backpressure) so if you can't isolate or locate it when running dry, try backing it down the ramp and getting it in the water and try again.