Hi, I've just signed up to this forum and I see a topic close to my heart, making a noise through a speaker!!
The 555 was the first circuit I learned and once I had that nailed I found myself getting into electronics, I'm still very much a beginner but these are great simple circuits to learn, and fun (and I love noise!)
I found the 555 quite limiting though as you won't get a true 50% square wave as the pulse width depends on the ratio of the two resistors across pins 6-7 and 7-8 (I think?!?!) you can end up getting a very nasly thin pulse sound if you are using a variable resistor to adjust one of these....what I discovered was the world of CMOS and the simplest oscillator ever, a true 50% square wave (to my ears anyway) using the 40106 schmitt hex inverter. The beaty of this is you can create six oscillators on one chip (mix them together or use one to modulate another, like a siren sound etc)...all you need is a battery, a 40106, a capacitor and a variable resistor (pot). The output is hot (ie. same as your power source) so you may want to reduce the output with resistors, but the chip can't drive a speaker itself (unlike the 555 which can drive a small speaker) so its best to plug it into some sort of amplifier, of course you can build one as well with an LM386 and a small amount of components (these will kill a battery very quick).
Not sure on forum rules for posting links to other sites but if it's ok, here are some links to what I'm on about that should help you.
The best bet really is to have a look at Nicolas Collins Hardware Hacking Manual, this has a load of CMOS circuits and good beginner electronic stuff, but the section on "worlds simplest oscillator" (page 64) will be what you are after!!
http://www.nicolascollins.com/texts/originalhackingmanual.pdfLM386 schematic for driving a speaker can be found here....this site also has a lot of schematics for building noise boxes using CMOS chips!
http://www.beavisaudio.com/library/LM386/LM386.htm