After many tries to get rid of that sound, I finally found a solution, and I want to share it with you guys.
Here is the pictures..(see attachments)
I was having very bad drone around 2100~2500 RPM on the freeway
and lower when driving local..
I did some search online and found this side branch resonator that works to cancel the resonance (drone)..
It uses the properties of sound waves to cancel out the drone..I found the ideal length of the resonator for my RPM range will be around 28 inches
but my muffler guy told me 28 won't do because of the space so the most I can get is 27 inches...I did it and it works great! although not all the drone at lower than 2000 RPM is cancelled, the drone is alot more acceptable and wont hurt my ears anymore!
so the solution to the exhaust drone is to have 27 inches of side branch resonator and let the sound waves bounced back inside the resonator cancel out the resonance!!!
Cost around $70 to install!! not so expensive