Does it only do it while the engine is running?
The little switch under the heater/blower controls is operated by vacuum from a pipe that connects to the inlet. It's a pathetic plastic pipe like a drinking straw and I've probably broke about four of these.
Push the switch in and see if you can hear a 'click'. If not, the pipe is either broken or a rubber pipe has come off!
You can start the car and stop it and push the switch in. If it makes a clicking noise from behind the dashboard, it's okay.
Makes me think the mechanic or whoever did the repair has left something unattached or broken this straw pipe.
"C20 LET bang"