narianu.... You don't describe a problem... just different noises.
if the bike is running the same as before... then there is nothing really 'wrong' with it.
The muffler might be cracked or loose... making the different noise... get down there and look close at it.. lay the bike over and look.
... and pay no attention to anybody who says to.. 'spray your motor down with water' to cool it... don't do that.
.. also pay no attention when he says.. 'its destroying itself'
.. if you are really worried... take it back to the mechanic who just worked on it.. maybe he forgot to tighten something ?