Oil leak and engine roaring noise

I taken to showroom that my vehicle timing belt has been cut off. so I have done engine head work in Skoda service center after service I run a few kms more than 1,000 kms after that my car had a oil leakage in injector seal and my car mileage suddenly drops very low pickup. then my car makes suddenly a noise like humming or it is like airplane crossing while I drive at the speed of 50-80 km. if I go in 80 km the sound is not very audible and if I drive low speed also no noise. but in between 50-80 I hear terrible noise then. also oil leak from valve cover near injector seal and
compressor air leak plus oil leak from glow plug. this all happens after I took the car from Skoda service center. no warning light, no engine light.vehicle run with no power.