Is it only superimposing itself on your vox waveform when you actually sing up from a particular loudness?
Looks VERY strange... No interference unless you actually sing. Is that what is happening?
It's nothing regular, not a hum or buzz, really looks like the mic got physically thumped.
 
You're not tapping your foot as you sing, are you?
 

nope, the mic stand is on carpet and im not holding it.   
Im going return the mic and get a new one and see what happens.

here is something of interest....
 
as I am recording, if you watch the waveform preview, it looks healthy!  (see here):  
 

 
 
but once you hit the STOP button, this is what you end up with:
 

 
this is the same take.   Just the first image is as I am singing, and the next one is after I hit the stop button.
 
I hope this may reveal something, i don't know.
 
Thank you!
Adam

What kind of floor is it?
 
The mic does not know that Sonar has stopped recording. Neither the mic nor the interface will behave any different.
If you sing, then stop abruptly, is there a tail of this low-frequency material visible in the recording?
 

jpetersen
What kind of floor is it?
 
The mic does not know that Sonar has stopped recording. Neither the mic nor the interface will behave any different.
If you sing, then stop abruptly, is there a tail of this low-frequency material visible in the recording?
 

its on a carpet floor.   there is no noise coming from the room.
 
I wonder if Sonar is actually doing something to the waveform because like it shows above, coming in it seems good in the waveform preview while recording, so why after the recording is finished, does it process the information to what we see in the end?

...oh, sorry, I understand what you are doing now.
Whilst recording, Sonar paints an "estimate".
When you stop, it then draws the waveform according to the real, stored data.
 
OK, so that is the waveform. Your vox superimposed over some really loud sub bass.
Well, I guess a faulty mic is a possibility. It just doesn't look like a faulty condenser or dry joint. It really looks audio/mechanical, not electronic.
 
Try with a simple live mic if you can get one. Shure 58, something like that.
They have built-in rolloff below 200Hz and they have no electronics, so if you still get this, it must be the interface or something else.
 
Edit: Yes, but the carpet is standing on - what kind of floor? Concrete? But OK, I'm sure if the carpet was on a wood floor, you would have said.

What happens if you disconnect the mic and cable from the interface, start Sonar recording and turn up the input volume slowly from zero to all the way open?

Also to try:
Set the interface input control to the level you use when you sing into the mic.
Then record with the mic connected but without singing, in a silent room.
Do you still get that waveform?

Could it be DC offset of some sort? I've never used that checkbox.

No. DC is a clear offset either up or down.
 
Let's eliminate things systematically one at a time, else we'll be guessing forever.
 
Please record some silence with your mic connected and the input level at what you normally use.
Do the same with the mic and cable disconnected.
 
If you get the low waveform only when the mic is connected, then it really is the mic.
Else it is your interface or some other source of interference.