2000 cougar...a couple of problems.

1. Low coolant light is on. I checked it and it looks fine to me. I've noticed (anytime I've drove it) that the temperature guage seems to stay around the halfway mark. Never seen it above or below. Perhaps a bat t-stat?

2. The car is very loud. I belive it has factory exhaust so I'm ruling out anyting like that. It's like a humming/rumbling noise. The faster you go the louder it seems to get. Once you hit between 55-65 it's almost unbearable unless you've driven it long enough to get used to it. The crazy thing is that once you reach 70mph the sound disappears.. Also when going around right curves (any speed) the noise is gone. This has to be something with the wheels/tires....right?

any help is appreciated

They have a sensor in the bottle make sure it is plugged in you could buy a scrap yard unit they give you 7 days for a free exchange if they don't work.
If the thermostat is working properly they normally stay in the normal range. If it never changes try a new sending unit. Use a copper wire and ground it by the sending unit the gage should peg.
You can ask a muffler shop to put your car on a lift and check for exhaust leaks. If you you do your own work put it on jack stands and look.

I'll take a look at that and see if that sensor is plugged in like you suggested. As for the other problem...if it were the muffler, would it only make the noise up til 65-70mph like it does?

also make shure your muffler or exhaust pipe is not touching you frame somewhere on you car causing the noise, this could cause the noise to get louder as you speed increases, or your exhaust mount could be bad,or even a bad motor mount

Coolant light...I'm sure it's on the NECO FAQs, but this one will do.
http://contour.org/FAQ/FAQ.php?s=probs&displayid=21

The noise sounds like a wheel bearing to me. Disappearing on right hand turns means its the left wheel bearing.

I'm with beyondloadedSE (http://www.automotiveforums.com/vbulletin/member.php?u=64119) on this one, probably a bearing or a bad tire tread.

Is there a vibration associated with it?
Does it get worse when you break (not including the speed change)?
When was the last time the tires were balanced and/or rotated?

Is the temp guage in the middle even when you first start the car? If it starts off at the low side and then goes up to the halfway point after it is warmed up then there is no problem. If it is always in the middle then take rhandwor's advice.

Horrible thermostat to change. If it is staying at half gauge even when car is first started, then you probably have a wiring or thermistor problem. Not reading right temp. Could be an air pocket in the cooling system, very difficult system to bleed. I use a vacuum tool to place the system in negative pressure and then fill. DO a resistance check on the coolant temp sensor and see if it is within range for the temp.

The sound is probably a wheel bearing. THe loading and unloading as you turn suggest as much. Possibly could be a seperated tread but leaning towards the wheel bearing.