I must have ever service manual they ever did but the excel, someone probably borrowed it.
You sure your not singing in the bath
and the neighbours whistling along.
Is that the model that has the different cartridges for different pressures, if so it could be that.
Otherwise i would flush it out and check the filter.
You can adjust the temp setting on the Mira's, to give more hot or cold, you used to take the body out and turn the gears, X amount of notches.
Has it got an identification code on the end face of the theroscopic assembly.
lt= low temp. ht= high temp