Petrol consumption problem

[andreas]

Half a year ago my nissan note 2009 started making a screeching noise when starting, usually on lower rev first/second gear.

As the screeching is in effect, the car drives slower, as if the breaks lies on.

I've had the aux belt checked, and the brakes. Mechanic didn't think it was the brakes, and tightened the aux belt. 

Screeching remains. 

Thing is, the fuel consumption is at least 50% more now, so something is not right. Also, the engine leaks oil. I fill it up quite often. Doesn't seem to affect the mileage or screeching sound though.

Is it something connected to the aux belt that is difficult to drive around, or maybe the drive belt? The mechanic didn't find anything obvious. 

Also, there is a big cabin sound on lower revs.

Any ideas?