Chatter marks can come from different sources. Most often they come from the splice on the abrasive belt. Belts are usually glued to hold them together which makes them a little thicker at this point. As the splice comes around, the splice is pushed into the wood by the contact roller sanding the wood just slightly deeper than the rest of the belt causing a deeper cut at that particular point.

A few solutions to this common problem are:

Make sure your contact roller has the proper rubber hardness (durometer) for your application. We recommend a durometer of 60 for most sanding applications. Harder contact rollers will take off more material but they are also less forgiving increasing the likelihood of unwanted marks.
Use a platen for finish sanding. The platen will sand less per pass and also spreads the sanding over a larger surface area because it is actually wider than the contact roller. Running your material through a couple of times during the finishing process with the platen activated should give you a nice smooth finish.
Make sure there is proper calibration between the contact roller and the pressure rollers to avoid hesitation and slippage of the stock.
Slow it down! Running your conveyor belt slower will allow more time for your abrasive sanding to remove the chatter marks.
Make sure to find a speed rate for your conveyor table that allows for the sanding belt to do its work and give you a quality finish. A way you can speed up the process is by sanding multiple parts at one time, spread them evenly across the conveyor belt.
Use a high quality abrasive belt with a well designed butt-type splice free of voids, glue and overlap.
In the event that chatter marks do occur you should try to eliminate one potential problem at a time by making only one adjustment at a time to your wide belt sander.