Mazda - Protege :: 2002 Engine Overheating / Rattling Noise Behind The Dash

I drive a 2002 Mazda Protege and it's been great to me until this past weekend. I had a 4.5-5 hour trip ahead of me and Google Maps told me a more direct route (i.e. not using the interstates) would save time. It must have erroneously assumed that the highway comprising basically the entire trip would be speed limit 55-65 the entire way. Instead, I found myself slowing down to stoplights and stop signs in small villages between 15-minute sprints. At some point, the interior heat stops - the air being pulled in from outside is not being warmed at all, no matter what settings I fiddle with. Not long thereafter I notice a rattling noise behind the dash. From inside the car it sounds almost like an animal skittering around in the chassis - irregular, short and clipped. With the hood up and an ear to the engine, it sounds like a piece of plastic fluttering in the wind, but with a faint electric "crackling" quality. Soon thereafter the check-engine light illuminates.

Before long the engine overheats and I pull over to the side of the road. Nothing appears to be leaking, and the crackling sound is the only thing noticable. Being in the middle of nowhere, I jumped back in and proceeded to drive at about 35-40, which was warm on the engine but enough to get me to a small town. At this point I checked the coolant reservoir and it was empty. Bought some from Wal-mart, poured it in, and headed out. Things looked fine at first since the engine had sat cold while I was running errands, but pretty soon it was overheating again at speeds as low as 30mph. After getting it towed to my destination, I took it to a garage the next morning. They couldn't find anything wrong with it, and indeed when I drove it back to the hotel that morning the engine behaved normally, the check-engine light was off, and the interior heating was functioning properly.

The next morning, when it was time to depart (but again after an entire night of sitting in the cold), a 20 minute drive to breakfast was smooth as before. Thinking that the mechanic had either waved a magic wand or I'd gotten extremely lucky, I hit the highway and did 65 for a good 20 minutes until the engine began to overheat again. Slowing down to 45 bought a few more minutes to find an exit, but eventually I had to pull over. Popping the hood, I notice the coolant in the reservoir is bubbling to the top! Thinking the mechanic might have added more and overfilled it, I cursed him in absentia. After a good fifteen minutes of cooling off, the coolant settled and in fact drained the reservoir entirely; I refilled it to the line and, again being in rural snowtopia with few options, decided to hit the road again. Not wanting to tempt fate, I stuck to 55mph and the car ran fine for an hour and a half (with the interior heat blasting, per a friend's advice). I stopped once to check the coolant and all was as it should have been - just below the line I'd filled it to earlier. Closer to a city, I decided to experiment again and took the speed up to 60mph.

This was great for 20-30 minutes but eventually the engine began overheating ever-so-slowly and I had to pull over, let it cool off, and replace another full reservoir's worth of coolant that had disappeared before I kept going. I drove the rest of the way home going no faster than 55mph and stopping on occasion to check the coolant. It overheated once even while doing 55mph, but a 15 minute break + coolant refill bought it the rest of the trip. In summary: Although not 100% correlated, going faster than 55mph seems to trigger the engine overheating, at which point the rattling noise appears and the coolant begins to bubble its way back up the reservoir (like it's boiling?!). I'm not sure how the coolant is disappearing because I'm unable to find any leak, nor is there any extra exhaust that would suggest it's burning.