motor spins

Hello I have a question, hope you can help me. I have a p206 with 60 ps. my problem started before a few weeks ago. and that is when the engine is a bit warm then it starts to become loud and quiet, it sounds like a hum,… loud, quiet, loud, quiet. this happens however only if I have no walk in it and let it roll or stop the clutch kick. Dirkt while driving when I give gas he sounds like always. and if I then stop with the car stops buzzing… almost only when rolling. it sounds like the speed is going up and down again, but I don’t know because I don’t have an ad for it… hope you know rat lg