Noise Machine

420 to learn

This is an excellent item to level your engineering! Unlike several of things engineers learn before this, it's purely dependant on cobalt (and negligible crystallised water/earth), which is considerably less farmed than saronite/titanium, at least on Doomhammer. Since you can learn it at 420, and it stays orange all the way up to 435 (and yellow till 440) you can save some of your much treasured saronite for other stuff.

Of course, if you're going for the Mechano-hog/Mekgineer's Chopper mechanical mounts, you'll be farming loads of titanium, and the byproduct of saronite will be huge - in that case, it might be better for you to use Mana Injector Kits up to 425 when they turn green, before you start making these.

Another good way to level your engineering if you don't want to do it the cobalt-intensive way is to create Mechanized Snow Goggles (http://www.wowhead.com/?spell=56465) - most guilds will have plenty of surplus borean leather lying around, and if you're not in a guild, it still sells fairly cheaply off the auction house.

materials needed

32 cobalt bars*
2 crystallized earth
2 crystallized water

* averaging 2 bolts per cobalt bolt combine