Did you replace the exhaust plate gasket, and does the exhaust mate well at both the engine and the base? The plate gasket acts as a noise dampener as well as a seal. The exhaust flange is usually bent and needs to be linished on a slight angle, so the inner is higher than the outer giving it a good seal. Also the base gasket normally needs 2 as there is always a gap. I often shim them then put the gasket. I have even sleeved the base so the exhaust sits tight and true. If all is sealed, then they are fairly quiet runners.
The fuel tank has a plastic cup that sits inside the cap, and acts ax a secondary barrier to stop the fuel escaping. You can make something up or use some cork, and put the breather hole in it offest to the cap hole, and it will stop most or slow it down, as their will still be an air gap between the cork and cap centre to hold the excess. Also using a half tank will help.
