Skoda Octavia MK2 - weird noise coming from engine area

[Costel88]

Hi all. 

Recently I bought a Skoda Octavia 1.9tdi which I gave it to a mechanic to do some work. I had to replace the expansion tank, new water pump, timing belt changed, alternator belt changed, new thermostat and changed the brake fluid too.

After he finished the works I get a really weird noise when vehicle is stationary but this noise comes only 15-20 minutes after I start driving the car after the engine and vehicle warms up.

If I start the vehicle from cold there is no such sound. 

 This is the sound that the car makes. Would be greatful if anyone knows where this noise is coming from as the mechanic claims it is coming from the compressor but prior to all these works there was no such noise.