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When I turn the key to the on position there is a buzz sound under the dash for about a half second then it quits and truck starts fine. Today i went to start it and it buzzed and quit but then i went to start it and the buzz came back, the tach was bouncing and the truck would'nt start. I shut the key off and tryied again and truck started right up. There was a thread on this awhile back but could not find it, wondering if anybody figured this out.
This is usually do to low battery voltage. When the volotage gets to low on the batteries the relays chatter, causing the buzzing sound. With your GP relay using a lot of power and trying to start it will cause low voltage on a set of "worn" batteries.
These are brand new batteries I just put in about 2 months ago. It does it when i turn the key to the on position, today was the first day it did it while cranking.
Check the voltage just in case your alt is starting to take a dump. Are you sure that the sound is coming from under the dash? Could it be the vac pump in the engine compartment and your hearing it in the cab?
Just went to check to see if i could hear exactly where it is coming from but now it won't do it. Both batteries read 12.4 with truck off and 14.1 with it on. I can hear the pump running and it sounds fine to me, so now I'm confused maybe it fixed itself?
Electrical gremlins are the toughest to chase down. Especially the intermittant ones. All I can suggest is keep an eye on it and when if it happens again try and track down were the sound is coming from. Like Lisa had suggested make sure the battery connections are clean and tight.