I need help.

Over 3 weeks ago I had all 4 belts replaced on my A4 1.8T (not quattro) as
it was going on 150,000 km.

About a week later - the car started making noises in third gear when it
hit about 50 km.  It is a loudish whining or humming noise and seems to be
coming from the passenger side under the hood.  I don't know much about
cars and called the dealership to report and was told it was just my turbo
and that older cars do have more engine noise in the cabin.

Since then friends have been in the car and have also hear the noise --
and as they also know the car they say I am not crazy and this is not
right.  Sometimes the noise seems to go away when I turn to the left
around a curve on the highway.  But it is loud and always there at speeds
over 50 km.  It is there with or without the airconditionoing.  I took the
car into the dealership frustrated and said to please check this and if
they had time to please also change the oil (which it didn't really need
yet for another few hundered km - but because leaving my car is an ordeal
- I decided to save myself time).

In the end the dealership said they hear nothing at an idle (never even
test drove it!!!!) but did change the oil.

They say that they do not "hear" things that are common engine noises that
other people think are problems.

I am taking the car back today -- and I am concerned that the same person
who changed the belts is going to be looking for problems they may have
introduced.

Does anyone have any suggestions to help?
I do not know about cars --- but before this loved my little black 98 a4.

R.