Any idea what material these gaskets are made of? Dealer claims they need replaced on my buddy's '07 Tahoe Z71 with 72,000 miles on the clock. It's been running Mobil 1 5W-30 for the past 50,000 miles and there are no leaks in the driveway. I'm going to get under the truck to see if it's weeping, but was wondering what material was used for these gaskets. They also claim there'a a "noise" from the engine after it starts up and they want to pull the thing apart. Not on my watch.