Help! Rats!

[Bev]

I've been fighting a rat problem since February, following a torrential rain storm. They have invaded my walls, ceiling and underbelly. I've had 4 exterminators out and 1 trapper. No one can locate their entrance. They've found nothing on the roof, underneath or around the perimeter of the home. Yet, they're there. The insulation is being destroyed, as is the fiberglass vapor barrier. I just had the 4th exterminator out today and I'm ready to pull my hair out from frustration. We've layed traps, bait, put a closed circuit tv camera under the house...you name it, we've done it. You never see them, but it's evident that they're there. Is it possible that they could be coming through the sewer? Apparently, they never get on the ground below the insulation because we can't find any droppings or rub marks, yet you can hear them going up the wall! If they were coming out of the sewer pipe above the insulation, wouldn't there be a leak? I'm at my wits' end with the problem. Any ideas about where they might be entering, sight unseen? (There are lots of stray cats around the area. They can't find them either.)