I will share what I have found.
After you disassemble your earcup, which ever side of malfunction, take a closer look into the plastic/vinyl cone.
There are two single copper wire going underneath the cone.
They are about or little bit thinner than human hair.
It seems like those copper wire rusted over time.
I used to wearing my QC3 while I was working out.
So, sweat and other moisture might be soaked into the earcup and eventually affected the copper wires.
The problem is not soley upon the rust, but it will be broken as the cone bouncing up and down million times and it ages.
I truly believe that BOSE intends to make this as it is.
Only two single copper wires, thinner than a human hair, connects between the circuit board and the voice coil.
2 years of life cycle... confirmed.
After 1 year of manufacturer's warranty, it will blow out anytime soon.
All the warranty/replacement items will be opened and the two copper wires will be replaced.
They will be factory sealed back, having a new ear cups, and sold as "Factory Refurbished".
I am not talking about the quality of BOSE products. I love BOSE
However, they should pay little more attention on those little things.
If any executive of BOSE read this, think it again.
Customers love a reliable quality product.
They will purchase it again for their loved ones.
Don't make a customer to buy the same thing twice within 2 years, or to spend more $$ for unnecessary upgrade.
After the inspection, I ended up tearing off the vinyl cone.
The copper wires were already broken before I tear off the cone.
The vinyl cone is somehow glue to the voice coil, so I broke the voice coil at the end.
My QC3 left is gone, permanently.
I had been using this for 3 years, and stored it for another 3 years.
Now is the time to get a new gear once again.
I WILL NEVER WEAR THIS WHEN I WORKOUT OR UNDER ANY TYPE OF MOISTURE ENVIRONMENT.
Hope this ease your mind.
Don't try to fix it because this is made as the copper wires are supposed to break after a while.