B20 making strange hydraulic noises

Hi, have an old B20 with no rear back hoe that is making a strange noise that sounds to my uneducated ear to be a pressure relief value opening somewhere around the seat. it sounds like a woosh (pressure increase), click (value opening) over and over again at irregular intervals.

i have no back hoe on this machine and there appears to be a pipe joining what i think would be the connector to the backhoe to a return line (circled in yellow on the attached). not sure if this is related or not

can anyone advise me?

hi, the FEL and 3 point linkage work like a dream

It does have a leak on the driveshaft out of the transmission that drips a little but the transmission oil level is about half way up the dip stick.

when i bought it i was told it had been recently serviced but no documentation etc so suspect this may not have been right so am planning a full service etc soon. do you know how much oil this machine would take? Also, is the filter a disposable one or one that can be cleaned?