Hello all,
Recently i've purchased a Ranger 98 and it has the original factory installed CD/radio unit. It's stock and nothing special but i plan to change it in the future.
I noticed this annoying PUP sound in all speakers when i power on the HU. Anyone familiar with this type of issue, any guidelines to start troubleshooting this?

Thanks in advance

 

Not uncommon in older audio systems, car or home.

Couple of reasons, none that would be worth fixing since it is too cheap to replace the unit.
Coupling-capacitors are often used on the power supply for the pre-amp and amp, if either fail DC voltage builds up and "pops", once powered up all is well.
Music/sound is just vibrations of air, speakers vibrate the air to mimic sounds, amps do this by varying the voltage to the speaker, the "pop" is a varying voltage sent out to the speakers.

I doubt car systems used speaker relays but home systems often do, these delay speaker connection for a few seconds after power up, avoiding the user from hearing the "pop" which is still occurring, lol, but not heard.
Sub-woofer amps almost always have a speaker relay, if it fails you will get a "thump" at power up and then another "thump" about 5 minutes after power is off, lol, doesn't effect sound quality, just annoying.

"Pop" could come from pre-amp or amp, the head unit often has both in a car system, so if you change the head unit no more "pop".
If you have a "premium" sound system it may have a separate amp, so you could get the "pop" even with new head unit if the amp is the part that is "popping".

Ranger radio diagrams here: Ford Ranger Radio Wiring Diagram

Don't see Premium for 1998-2002, but diagrams could just be missing.