how many miles?
have you recently had service done?
do you have four-corner load leveling?
From my experience ('my '97) suspect you have a leak in either rear bag(s), front strut(s), or possibly both.
There is also an air union coupler under the master cylinder that is prone to fail.
if original pump, it may be on it's last legs. mine got extremely noisy before catastrophic failure.
Again, my experiences with my '97. you'll have to verify.
let us know what you find.