The most likey thing that causes this is a noisy thrust bearing. If the noise disappears when you depress the clutch, then this is definitely what it is.
It won't affect the MoT, but it is a sign that your thrust bearing is on its way out, which means gearbox off and replacing it. The part won't be that expensive but the labour will, as it is intensive work. If the bearing goes, you won't be able to use the clutch at all, so no go for the car.
Retired traffic police officer. Vehicle examiner.