Don't know if this will be any help, but my 2000 did the same thing sorta, squeaked when I would especially slow down for speed bumps, etc... I tend to believe it was my center caps...unfotunately they were stolen...but no more sqeaks at low speeds.Also try lubricating your strut bushings...3M makes a lubricate for rubber parts that you can spray on just like WD40..the hard pounding noise maybe a more serious problem..
maybe a ball joint worn out or a wheel bearing. jack the front end up and lock the steering wheel up. wiggle both wheel up and down or back and forth. if they move more than a centimeter than chances are your bearings are bad. also look at all the rubber pieces on your front end and look for any that is torn up
You will notice this a LOT more with an older car such as a 65 mustang.
When it rains (or a week or so after) the moister will get inside/around the rubber bushings on the control arms and when the rubber bushings are flexing and rubbing the moister is what is causing them to squeak. It should stop when the weather warms up but if its really pizzing you off just get some wd40 and lub the control arms.that should fix the problem