Toyota Repair: transmission plug sizes, popcorn seeds, tranny oil

Question
Hello!  I have a 1996 Toyota Tacoma 2wd, 5 speed, 4 cyl with 213k miles.  Engine 2rzfe..I believe the tranny is the W59.  I am getting a weird sound when I am in 3rd gear and 30 miles an hour, 4th gear at 4o miles and hour and 5th gear at 50 miles an hour.  If I am below or above those speeds in those gears there is no noise.  It isn't a grinding noise, more like a muffled rattle or like a few popcorn seeds rolling around in a glass jar.  I am going to try to drain and fill the tranny oil first as I saw in the toyota manual that noise could be caused by low oil levels. (This was my daughters vehicle while in college and now I have it for commuting back and forth to work (about 140 miles round trip!#.  I picked up some Penzoil gear lube that states it is good for GL-4 and 5.  I also picked up a couple drain plug grommets for the drain and fill plugs #screws).  Do you know what wrench I need to get the plugs out?  I know they are metric and I don't believe I have one big enough and don't want to buy a whole set etc. etc.  Additionally, if this doesn't correct the noise issues, do you have any idea as to what it might be?  Does the timing chain make such noises?  I just had a tune up done and they didn't say anything about the chain...not sure they even looked at it but I heard the chain makes noise when it is going to need replacing.  Thanks a ton for any help you can provide.

Answer
If the noise was coming from the engine it would be there when revving the engine up without the truck moving. If the noise is there while driving only then it would be a driveline component noise,including the transmission and differential, so first determine if the noise is there while not moving the truck and revving the engine to about 2,200 rpm by slowly bring the engine speed up. Let me know.