that`s just water velocity noise mate that your hearing. As the room warms up and the valve starts to close reducing the diameter that the water is being pumped through. Fixed speed pump and an ever decreasing orifice its inevitable that you will get this noise.
Later type energy saving/ variable speed pumps can help.
Usually the bathroom radiator does not have a TRV as you need an open loop on the circuit for pump overrun. This is to stop the boiler overheating unless there is an automatic/ manual bypass fitted.
Just mentioning this in-case you experience issues after you have turned the valve down a bit. Then again you may have another rad on the system without a TRV.