Have you had that engine a long time? I ask to base current performance to the past. Has the engine always had the same burble? Have you always run similar sized props? What kind of RPM does it turn the 26 -10? My guess is quite fast if it is ripping. A 26 inch prop is small by 100 twin standards.The fix (or reduction of the problem) might be had just by going to a slightly larger prop to increase the prop load. The carb might be an issue and a change or rebuild might be in order. Playing with the pop off pressure might help but at the expense of time spent that may not pay off with any improvements. Lastly, that might just be a normal characteristic of that particular engine and exhaust system.

Keep in mind that burble is just the engine running a bit rich at a particular throttle setting with a given load. Usually when the engine is burbling in level flight, just raising the nose will reduce the burbling w/o touching the throttle. A larger prop size has the same effect.