Its where shocks are mounted to the frame, open hood and you can see them on left and right side just beside fender area (i think the centre nut holds the shock and 3 other nuts hold it to frame). oem shock mounts are pretty expensive as they contain a bearing along with rubber etc.
I may be wrong but my guess is that whenever there is dug dug sound, it has to be some sort of mounts issue. Feel free to correct me but from what i have heard early stages of failing shocks would make chun chun sound when you move it with hand at stand still. A later stage of failing shocks would be that shocks do not oscillate enough once compressed or oscillate too much before settling down.
Diagnosing suspension problems is not too easy so we cant do it ourselves on the other hand most mechanics do not bother to check and investigate properly, they go for easy way out which is usually the expensive way 
