Ok, you are getting way ahead of yourself and did not even answer my 3 simple questions which will tell us the most.
1. Have you been loosing coolant?
2. Is there oil in the water?
3. Is there water in the oil?
There is no such thing as warped pistons (that I know of). However since you mentioned it, there is such thing as "piston slap" when it's cold out, I don't think that it lasts for 20 minutes. You will have to google that yourself; us Phoenicians never get that.
This is what happens (will answer your "heater working" question):
Let's say I wake up at 7am. It's 25 degrees out (snow last week).
I start the car up, and my car will sound like a diesel engined semi-truck. I think this is from a leaky gasket, personally. I then back up from my parking spot (the sound of "diesel" lessons a bit when I do this, actually), an then I will start going.
Ok the first time I read this I was thinking you meant rough idle. If it is purely sound, it is probably a small leak at the header gasket or a bolted exhaust joint. It could be going away when the parts heat up and expand and close up the leak. Also maybe condensation overnight is causing more pressure in the cats?
After driving about 5-10 minutes going ~40-50 MPH, the heating gauge will start going hotter an hotter. It eventually reaches past the "L" in normal, and sometimes even touches the Red Bar all the way to the right. At this point, my vent will not blow hot air from it. When I put it to cold, it blows the same temperature as if it were hot.
After those 5-10 minutes, I usually get on the highway as I use it to go everywhere. I will stay on this highway for another 10 minutes. Sometimes I find that when I try to accelerate on the highway to reach 65-75 MPH, I will reach 4k RPMs and I can feel it not shift to the next appropriate gear. Finally it does. At this point, the heating gauge remains past the "L" in normal, but sometimes it then actually goes back down to the "N" in normal on the gauge. It will remain like this as long as I go 70-80 MPH. As soon as I get back to 45 MPH, it will have that same problem. And during the times where the heating gauge does go back down, my Vents will blow cold or hot air appropriately. Usually that diesel sound will stop after 20 minutes, even with the heating gauge situation still taking place. This diesel sound will then remain gone for the remainder of the drive.
Again... please tell me that you at least checked your coolant level!? The problem you describe could be a number of things. Most likely would be bad radiator cap and/or bad thermostat not allowing the coolant to circulate properly.
So, from that, what exactly would cause that to happen?
From my own perspective at this point, I think I have:
-A leaky gasket and that's what is causing the diesel sound.
-Damaged or Blown Head Gasket.
Things I think I might have:
-Clogged Radiator Buy some radiator flush and use it. Refill with 50/50 antifreeze and distilled.
-Thermostat not opening until it built up pressure (explains why I need to go 70-80 MPH for my car to cool?) Install new tstat and cap to see if that is it. They are both cheap to replace and both fail under normal use after time.
What I'm not sure though, and probably my main questions would be:
-Can the damaged or blown head gasket also cause that temperature gauge situation to occur? Possibly, if you are loosing coolant and have air pockets it can cause this. You do loose a little coolant over time under normal use though. Or is it my radiator? Unlikely. Have you ever put tap water in there? You should only use distilled unless it's an emergency. How would I know which is causing it? I'm not sure. Use the flushing product like I said and see if a lot of crap comes out or if it's pretty clean.
-Can a thermostat cause those problems? Yes
-I don't think I have warped pistons if eventually the noise goes away in my car, right?