as sAro mentioned, warping is probably the most likely offender. The wobble becomes more pronounced as you increase the tempo gap - for example, if you have a 130BPM track or sample and it's warped to a master tempo of 133 or 134 (or the other direction or whatever) and the warp mode is "beat" that tremolo-ish thing is really apparent. One other thing that you can try is setting your sample to "Hi-Qual" in the sample window of clip view (right next to the warp settings). It won't make the wobble go away but it allows Live to do higher quality dithering of the sample. "Tone" or "complex" are your best bets to mask that wobble. Unfortunately though the math and technology isn't quite there yet to be able to warp too far off true tempo without noticing some kind of wobbly/tremolo as an unwanted effect.

If you're mixing trax - another thing you can do is assign a controller (like pitch bend or an assignable knob) to master tempo and adjust the range in the midi assign window - that way you can have control over the master tempo and fluidly move through the mix. It's a little more work but it helps. It is also possible to assign BPM changes in the master scene launch column.