Check that the headers are set right. If not, then they can cause the exhaust pipe near the end of the muffler to hit the body. Everything flexes down there and it goes away once the engine is running at speed and smooths out the vibes.

These had to be reset since you had the Centa unit put in which meant that everything had to go, including the engine, compensator and trans.

Just a thought. Mine did it after I had the Centa unit replaced as well as the header which broke.

What's your mileage? Who cares. Is it practical? See #1. What happens when it rains? You get wet.