D4G vibration

I have a d4g and I have been noticing a vibration at idle. This is a little elementary but its the only way I can describe it for you. So it goes, viiibbrAATTION smooth viiibbrAATTION smooth. Think of it as a clock. It starts at 12 lightly and gets worse till 6 and is smooth till 12. Its about about two seconds per cycle of vibration. As you speed up motor it gets faster, then smooths out about 1/2 throttle. 1300 hr machine. Any help would be great. Thank!

D4G vibration