What the knock was that?!

[wildblu]

1999 Tahoe 116k+ miles-I was driving down the road today as I often do when I'm driving and I heard and felt a thump which seemed to come from under the floorboard of the the drivers side. At first i thought I had gotten a flat but nothing in the steering seemed to change. The temp here dropped to the single digets and snowed so I thought I had hit an ice chunk, however it sporatically continued through out the day. I have no problem with going into gear. I checked trany fluid and its fine. I looked under the rig and the normal leaks are there (little oil here and there). My oil was a might low which I will take care of in the morning. I am concerned that this is the begining of a larger problem and if anyone has any insight as to what might be going on I would greatly appreciate any input. Thanks