DB07P1

i recently just got my car back from a garage after having it fixed due to blowing my head gasket and water getting into 1 of my cylinders,whilst in the garage the mechanic noticed an oil leak which was dripping onto my manifold,so he fixed it,only i got the car back and after having it 4 30 mins it had to go back because the oil leak started again.so he fixes it again and puts a new rocker cover gasket on.
iv had the car back for 4 days now but sumthing isnt right,there is a strange sqeaking noise cuming from the engine every now and then,sumtimes its worse then others,it will do it when its idling and sumtimes wen im drivin at low speeds,and all in all the engine just doesnt sound as clean as it did before.and when fully warm the engine is ticking over at 400 revs sumtimes lower,almost as if the car could stall,but it never does,bearing in mind the engine had to *** out to fix the head gasket,i am worried when it was put back in the job hasnt been done right.
does anybody have any ideas as to where the problem may lie??