There are a few things up front that can cause a rumbling sound. It does depend on whether the rumble is directly related (in frequency) to the engine speed, or to the vehicle speed.
If it's related to engine speed there are a number of areas to look at. Wouldn't expect the turbo to do it, it's spinning at 75 times faster than you can imagine (actually, somewhere around the 100,000rpm mark). It will whistle before it rumbles.
It's worth investigating the air filter's box to make sure that it is sealed. My own airbox broke the side cover and it sounded a bit like it was throwing the timing chain - it was just induction noise modified by a hungry turbocharger.
Check the blades on the cooling fan, particularly the root of the blade - look for cracks. If the blade can flex when the fluid couple drags on it hard, not only will it make an awkward sound, but it could tear a large gash across the back of the radiator.
It shouldn't be the alternator, never heard of any problems with the V9X alternator, but that's something to consider. If you have some means of adding a substantial load to the engine (got a winch?) it would be fairly easy to test. Otherwise, slipping the serpentine belt off and turning the alternator over by hand might reveal the problem. It should feel a little heavy (like there's weight on the shaft) but it should turn easily and smoothly, with no noticeable wobble in the shaft.
The water pump (again, not a common fault) could also cause this noise, particularly if there's air in the coolant (cold engine - check coolant levels). While you've got that belt off, check the water pump.
It's pointless trying to check the fuel pump. Not only is it driven by an internal mechanism, it drives so hard and fast that if it made noise, it'd be a squeal rather than a rumble (a bit like the turbo).
If engine performance is down, it could be a boost leak and it'll show up fairly easily - I have one in my engine bay right now that's not making noise or affecting performance but I've got a mess of dirty black oil sprayed all around the baffling that's just outside the turbocharger. It's a big mess - you'll know if you have a boost leak.
Take a look at the top of the intercooler. There were a couple of people that had intercoolers where the hotter region ballooned - it's worth checking yours.
On to the drive line. It could be bearings, uni joints, drive shaft - the front shaft has to be installed correctly. I'd be more suspicious of the bearings than anything else but that'd also be a first for a 550 that I've heard about. The YD25-powered D40s have a history of poor front-axle bearings.
Unis are easy to check, movement = bad. Because your front hubs aren't manual or auto-locking (the front diff is constantly turned over by the rotating wheels) the drive shaft is in motion regardless of whether you've selected 4WD or not. The transfer case works by engaging the input shaft by shifting a pawl across to link the gear that meshes with the chain that is connected to the front axle output. This means that even in 2WD mode, the transfer chain is being turned over and if yours is stretched, it will make loud noises. Driving on firm surfaces in 4WD will destroy that chain - not quickly, but definitely over time.
The front diff is not the hardiest on the planet and it's actually a sad surprise that they didn't put Titan diffs in the V9X. It's an open diff, so it won't be a problem with the LSD plates, but if you've had one of the front wheels spinning and then suddenly biting firm ground, you may have chipped a tooth in there.
Following on from the front diff, that sort of activity can snap axles and tear the spider gear off your CVs too. These are easy to check - hoist the car and listen while the wheels are being turned over. A ruined CV may cause a complete disconnection from the drive train - on a hoist, turning the left front wheel in a forward rotation will cause the right front wheel to reverse - if that doesn't happen, look at the CVs. CVs can become noisy if the boots are damaged and debris enters, so a close inspection is called for.
The last thing that it could be won't depend on vehicle speed and MIGHT relate to engine speed but will certainly depend on the amount of pressure you place on the steering wheel and that's the power steering pump and the rack. Next time it happens, steer the car a little and see if the sound goes away.
Hope that gives you a few options!
Cheers Geoff Go the V-6's
