Polo 1.4 cl (1997)

Hi
I recently purchased a Polo 1.4 cl 5 door hatch, very pleased with the car.
just noticed though that when you are in 5th gear and take your foot of the
accelerator there is a very quiet (hardly noticable) sort of a whine as the
engine revs drop. it does not make this sound in any of the other gears,
also does not make the noise when driving along, only when you take your
foot of the power in 5th. Is this anything to be concerned about please.
Also can anybody tell me how often (miles) the cam belt needs to be
replaced. mine has 37000 on the clock at present and as far as I know, it is
the original belt.
thanks very much
Roger



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