Where I live, some parking lots have dips in them when you leave. Occassionally, I will hear a loud scraping sound from my 1990 240 Volvo DL when I go over these dips. The other day I had my friends in my car and the scraping was more often. What exactly is getting scrapped underneath my car? I'm scared it could be the gas tank, but the mud flaps sit lower so it seems like it should be the mud flaps.

 

could be the flaps.... have a friend stand out side nexttime and see....

If you don't do this already, start.
Take the dips at an angle.

 

Like TIPSP said. Probably your mud flaps, Or it could be your tail pipe. I highly doubt it's your gas tank unless you are really trying.

 

What is scary is that I looked underneath and I saw a few scrapes on the gas tank. But I just bought the car a couple of years ago so I have no idea if they are from me or not. Ether way, should the mud flaps be scraping the ground? Is there something wrong that I need to fix or is this a common trait for 240's?

 

If it's the back, it could be the spare tire well but I vote for the muffler. Check your hangers that it's not bouncing down and like TIPSP said, have someone else drive while you watch.
or you could do the Click and Clack method of tying your brother under the car for the test drive.

 

Most likely it's the tailpipe as I had the same problem with mine pulling out of my girlfriends driveway. If I went real slow it wouldn't happen because my shocks are worn out and with a little speed the suspension compresses more. I cut a couple of inches off the tailpipe and it stopped. However the exhaust was now blowing into the back bumper and coming out the side of the car on the quarter panel and leaving a soot mark. So I went to autozone and bought a 4 inch tailpipe extension. Screwed it on so it extended just past the bottom of the bumper and problem solved..