I'm thinking maybe a WD-40-fed drip-irrigation system might be what's called for. If anything, my engine & suspension would get cleaner.

I had to abort my last attempt to diagnose (rain) but before I quit, I managed to squirt some far-too-expensive waxy-preservative lube into the lower ball joints - unhindered by any troublesome rubber boots. Joggling the frontend afterward was rewarded with the same squeaky sound, but I wasn't sure the lube had gotten in where needed yet. I moved on to try to get some of the same stuff into the sway-bar bushings, but it was probably too thick to make much of a change...and then came the rains.

There's a deep springy resonance to the squeak that suggests it's from two surfaces that are 'traveling' a longer distance with respect to each other than any ball-joint - ie one of the sway-bar bushings - so I will squirt some WD in there when the next opportunity arises.

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'83 300SD - daily driver
'97 Dodge Cummins
'60 & '64 Minis
...etc...