ive just had a new heater core fitted in my escort van but wen i put the heater on it only runs for a bit then makes a buzzing sound then goes of. if i wait 20 mins it comes on again then goes off can any one help

 

Sounds like the blower fan is overheating and then jamming up,surprised there's no fuse blowing though?

Double check that nothing like leaves or anything else have got jammed in it as it sits right under the scuttle panel up top.

 

Does sound like the heater fan getting leaves or similar in it.
tabetha