miztawizord

sep guys, 1st post. I'm a new member to this forum and owner of a 3 week old '93 sc400 w/167,000 miles. besides the usual problematic things with the sc's mine makes an unusual sound when going over 40mph. there's like a loud rumble under the car when you accelerate and when you get off the throttle its a little louder? I'm very puzzled by this. any suggestions. don't thinks it's the tires or engine. I have full power, I guess from the 3 weeks I've had it. this just started 4 days ago and is getting noticably louder. can it be an exhaust leak? has anyone had this problem before? thanks. sorry moderators if this is not the right thread to post. I searched and found nothing. *must note* car is stock except for 20" rims and they have no uneven tire wear.