low pressure, when handle released hum hum noise?

It could be a simple dirt/rust problem in the hose, Trigger or lance. Give them a good cleaning inside and check the nozzel for dirt and rust also.
The other reason could be that the unloader valve is sticking. If the unit has not been used regularly, rust can build up within the pressure regulator valve and in the units water pipes. Try turning the pressure knob, up and down to see if it makes any difference. If it doesn't, the valve needs checking for rust and cleaned. Also, check the rest of the water section e.g. outlet pipe etc. and again, clean any dirt or rust.
If the unloader valve/regulator still does not work it may need to be replaced.

Davy Mac
March 2009