Honda civic 1.6i sl auto

Hi i have had this car now for almost 3 months, it has 100.000 miles on the clock. When i 1st brought the car it felt very smooth and a
perfectly good car.
Just recently done a trip down to cornwall in the car 400 mile round trip to and from my house and started to have problems with this car.
The problem i have is that once it gets to about 4000 revs whatever gear it is in it makes a horrible sound like there is a plastic bag caught on the underside of the car almost like a whistling sound. It is worse sounding in the higher gears and i have lost a lot of power, i am struggling to do more than 60mph on the motorways. It is fine around the town below 40mph.
It almost sounds like the exhaust is blowing but when i stop put it in neutral and rev the engine it does not make this noise i am talking about..
I dont know much about automatic gear boxes as this is the 1st automatic i car i have ver brought or driven....
Any help will be much appreciated..
Many thanks and look forward to a reply
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