1995 5.7 Chevy Silverado - humming noise near throttle body

The humming noise may be a vacuum leak. The metal hose might be leaking and vibrating around creating the noise. To check for vacuum leaks after you narrow an area down where it sounds like it is coming from, get a piece of sewing thread or light string. Drag it around the seals, hoses, etc. and it will get pulled towards the leak.
The thud noise could be a the exhaust, it all depends on how solid and loud the, "Thud" is. Other than exhaust, I would suspect motor mounts.
You can check this by opening the hood with the engine running, have a friend watch the engine. Put the parking brake on and hold the standard brake while you put it in drive and blip the throttle, see if the engine moves excessively or if you replicate the, "Thud" to any extent. You can also use this method to see if the exhaust is loose. Just have someone looking under at the exhaust when blipping the throttle.
I hope this helps.

I don't have any friends :( Just playin. Thanks for the reply. Are vacuum leaks serious? Is it expensive to replace motor mounts or are these something I can just tighten?

Vacuum leaks can be serious, it depends on how bad they are. Bad ones usually cause the idle to be erratic or high. If it is not doing that it is not top level serious, but it still leans out the air/fuel mixture and lead to problems later as they only get worse.
Motor mounts usually need replacement as the bolts holding them in place are lock-tited and torqued to the point that I have never heard of any coming loose. The have a rubber isolator that suspends the motor to isolate vibration. After years go by the rubber gets hard and brittle and they fail. Usually the torque side of the motor fails first and the other side fails soon after as it is bearing the entire load of the motor. Then the motor can end up resting on the frame and out of alignment leading to bad problems.
Definitely check into the mounts as they are not hard to diagnose.

Well i looked all over for an answer to my next question...so I figure I'll just ask you....what is the idle speed for a 1995 chevy silv 5.7l... mine idles at about 900 in park and around 600-650 in drive. is this about right?

That sounds correct. So that humming noise might be that loose metal pipe vibrating against some other metal piece and causing the humming noise.
Have you got under the hood and tried to locate the area where it comes from?

could be...i'll post pics of the metal hose I'm talking about...might give you a better description...thanks for helping out by the way.

You're very welcome. I will keep an eye out for them and reply ASAP.

ugh, been raining all afternoon so I haven't had a chance to really get outside and take pictures for you. i was working on my '04 aveo earlier this morning when it was just sprinkling...actually i should have researched this a little better before i did it but... what if a little bit of water gets in the timing belt area...I needed to adjust the tensioner on my timing belt because it was slipping off the cams. I put an umbrella over the area i was working but i was worried a little water may have still got around the area...may also be some splashing from below... i have both timing covers off too.

The timing belt area is pretty much okay as long as major debris does not get into it. Unlike a timing chain, which is sealed inside the engine, a timing belt is, "outside" the motor like accesssory belts. The cover for the timing belt is not an airtight seal is it?
It is usually just a plastic cover that keeps debris out. Moisture can form from condensation as the motor cools down.
So, I think you are fine and a real trooper for fixing your car in the rain under a tarp.

So...I didn't mention this but when I bought the truck there was engine coolant that was "caked" on two lines coming out of the engine so I took it to a mechanic to have them look at it. Today I discovered that the oil cooler lines that the mechanic "fixed" were still leaking. i called another mechanic and he told me with 95% certainty that there was a crack on a flange in the radiator and that was causing the leak. he said he's seen it a million times in the silverados. he said i need a new radiator and that i wasted my money on the other mechanic. after researching these oil cooler lines I realized that they also are important for the transmission. I looked under my car and saw coolant pretty much all over the underbody and what I thought was the transmission...so I guess my question is... How screwed am I? These lines were probably leaking forever before i bought this truck. The thud i mentioned earlier is more like a low pitched rattle...like the truck is lurching forward or something...I crawled under the truck and tried wiggling the exhaust to see if the exhaust was loose like the car had suggested when I bought it...no movement. Could this be my transmission? It only happens when I first take off... I'm so confused and ****ed off... II think I have made such a huge mistake buying this truck.

Vehicles are a pain is the ***. Sometimes ignorance is bliss. When you have some mechanical knowledge as we do, every thing that is wrong bothers us and needs to be taken care of. Most people drive until the vehicle fails. We repair until the vehicle fails, we have a nervous breakdown and are broke while the other blissful car owners' have money to get another car.
Actually I am kidding...$100 in maintenance is $1000 in repair.
Hang in there, it is frustrating and maybe you need to take a break and get away from it for a bit. It helps me to do that sometimes.

Okay, as far as the, "caked" stuff on the radiator lines, I am assuming the upper radiator hose has a whitish substance caked up around the connections.
This is indicative of cooling system issues. There is a quick test you can do. After the next time you drive it, grad the upper radiator hose and squeeze it. You should feel some resistance that makes it hard to squeeze it till its' pinched off.

As far as the lines going into the radiator form the transmission, they are coolant lines for the tranny fluid...you know that. The transmission depends on pressure to perform correctly. Also, check the fluid level. With the engine running and at operating temperature, put the transmission in reverse and drive a few times and then in park. Then check the level, leave the engine running while you do so. Let me know if the fluid is brown or smells burnt.
You might be able to get around a new radiator.
If you could, It would be easier for me to reply to your reply to this message if you e-mail me.
My e-mail is [email protected].
I will reply ASAP.