I'm pretty sure it comes from the two glycol filler holes. These screwholes are drilled out and its the bare aluminium there that reacts with the glycol.
If you look closely at the holes through the front of the tubes (lens needs to be off), you may see small silver bubbles in the screw holes - thats the aluminium reacting with the glycol. At first I thought it was the iron screws but having replaced these with stainless steel ones, I popped the old iron ones into a glass of old used glycol to see what happened. Other than fungus that appeared (which I suspect was in the old glycol anyway), I have seen no adverse reaction from the old screws. The amount of garbage that comes out of these screwholes over time is incredible, enough to leave dark 'A' shaped streaks on the tube face as well as all the blobs that sit on the bottom of the chamber.
Two solutions for you, lower the level of glycol in the chamber to just below the filler holes so that the glycol can't react with the filler holes (not possible if pj is ceiling mounted), and/or put some silicone in the filler holes before putting the screws back in so that it coats the inside of the filler holes.
The only way sure way to get rid of the impurities is to drain the glycol, dis-assemble the LC housings and remove the C-Elements to get at the chambers, throughly clean the chambers and the tube face with Isopropyl Alcohol (spelling), use distilled water to clean the back of the C-Element. Leave to dry then re-assemble using silicone to seal everything back together again. Not easy but do-able, BUT you run the risk of damaging the bellows in doing this and replacement bellows are now all but impossible to get so this may no longer be a realistic option (difficult NOT to damage bellows when removing the C-Elements as they tend to be very well silconed in - I damaged one out of the three I cleaned). I think some folks tried flushing out the chambers with the bellows/C-Elements in place (i.e. putting cleaning chemicals in through the filler holes) but that will only deal with fungus, it won't address the filler hole problems nor clean the tube face properly.