Noise when coasting

I am kind of new to this vehicle. We got the car on May 2nd of this year, and it was a brand new 2008 model. It has been driving wonderfully these past couple months, getting great gas mileage and had no problems with it. Just yesterday, I noticed a change in the way the Prius handles when coasting. There is a noticeable lower pitched humming and extremely rapid decelleration, as if breaking, when coasting. Before, when coasting, there was very little decelleration, unless on a road that goes slightly uphill.

We are taking it in for an oil change this afternoon since it is at the 5000 mile marker, as the maintenance schedule says. Not sure if this might have something to do with the oil filter or perhaps there being a low amount of oil. We are not taking it to a dealer since the nearest one is 75 miles away, so I don't know how much the technicians are going to know about the Prius. I doubt if they would have any idea what's going on.

I read earlier threads, but the closest one I saw to my problem is when someone's M2G was failing - which was said to be a $5000 repair. Isn't my car a little new to be having that happening though?

If anyone has any suggestions or ideas, I would greatly appreciate it!

 

The guage appears to be working normally. I haven't heard the noise again since that morning, so maybe I did just accidentally knock it into B instead of D. Thanks for your reply