Hi mate,
My friend (is writing this) had a 172 for 5 months, 28000 miles, great condition... bought it from a main dealer with 12 month warrenty, however;
from the 3rd day i had it, it whistled under acceleration, at first only a bit, but then gradually getting louder. Dispite the warrenty and days and days in the various garages, no one could establish the problem. I had the cat conv and clamp and all of these things changed.. and even renault were contacted. No one could find the fault - and it drove me nuts in the end. The dealer gave up and put it doqn to a trait of the car rather than a problem...I ended up selling it back to the dealer at a £2,200 loss (after trying to go down all kinds of legal routes) as no one would look at it through a private sell.
The noise got worse once the car was warm... and was especially bad when people were in the back seat.
It came in above 5000 revs, so was really hard to show engineers without scaring the shite out of them.
If i can help at all just p.m me...