I was probably the one that suggested that you were hearing a relief valve. The issue is that if you have the transmission in your mower that I think you have, it is not really considered a repairable unit. Although the parts may be available on the internet, the best repair may be to replace the transmission. All the internal parts are set up at the factory and I don't know how to tell you to adjust the relief valve(s).
You did confirm that you had plenty of oil in the transmission, didn't you? And that it is mounted firmly in the frame, so it can't twist and slack the belt.