Yours is coming from the front of the engine (timing belt area), correct? If so, it could be one of three things, water pump, idler/tensioner bearings, or dry bearings on one of ur accessories (alt, PS pump, etc). I'd place my bet on the bearings. It's quite typical for them to age and become 'loose', per se. One more thing, how old's your timing belt? If new, it could be over tensioned. If you decide to replace ur tensioner bearings, do both plus the water pump and timing belt (preventative maintanance). How many miles are on ur Miata?
Side note: I'll be away for a week for work so I may not be able to post any more on this unless I can find access to a computer.