Hi..I have sinusitis and the plugged-feeling eustacian tubes and firecrackers in my ear too...I used the valsalva maneuver often too but read that it can damage your tubes if you're not careful. I don't know if that's true but I've had the firecrackers for at least 10 years.
It is likely that the sinusitis and the problems with your ears are directly related. The inflammation from the chronic sinusitis has probably caused some swelling of the opening of the eustachian tube. This causes the sounds and discomfort you are describing. The treatment for this is to get rid of the sinusitis. Sometimes, when the ear problem is really causing discomfort, a short course of oral steroids will help. This is a last resort. I would continue to work with your doctor to get the sinus problem under control, first.
i'm sure there are many things it could be...but my mom had severe problems with her eutashian tubes and it was later determined this was from undiagnosed hypo. hypo caused fluid to build up in tissue in the ear. it also had affected her hearing. she heard those crackling sounds most of her life.