My original air pump went bad, it started taking 6 - 10 seconds for the 4WD light to come on.. and under medium - heavy load, the front diff would start slipping..

Listen at the 38sec mark..

Suzuki Grand Vitara Offroad 4

I replaced the air pump with another used unit from a crashed Grand Vitara. Now the 4WD light would come on instantly, but it blinks intermittently and you can hear the pump make noise every time it blinks..

I haven't tested the vehicle as yet to see how if the front diff still slips, but what could be the cause of the intermittent blinking / hum from the system?

I checked all the rubber hoses and they seem to be in order..

I disconnected the hoses from the pump and plugged the pump's outlet with my finger, the same intermittent problem occurred.. Would that suggest a bad O ring inside the pump?

Info on the Pump's Orings here..
http://www.suzuki-forums.com/1g-1999-2005-vitara-grand-vitara/35587-you-4wd-compressor-bad-just-oring.html

It could be bad sealing, causing loss of pressure, and making the pump run to restore the pressure.  A leak could be O ring, or inappropriate release (say a leaky seal) on the release valve.

It could also be caused by momentary loss of 4WD signal, either from the switch in the TC, or within the 4WD controller under the dash, or an intermittent electrical connection.

I plan on opening up the pump and changing the O Rings.. fingers crossed that is the problem and nothing with my front differential..