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