'99 NBS fuel pump

[53burb]

I have a '99 NBS 1/2ton 4x4 with a 4.3/5 spd manual tranny. Anyways, the fuel pump was replaced a couple of years ago, NOT by me. It was fine at first, then noticed the pump humming shortly afterwards. I finally pulled the bed off about 6 months ago to look at it. The lock ring wasn't on correctly & the pump was loose. Put the lock ring on correctly, problem was fixed...so I thought. It's now making the same noise again. I haven't pulled the bed off yet, but I'm sure of the problem. Anyone ever hear of this?? One guy my wife spoke to about this said it was common in GM trucks, but I "googled" it & didn't see anything. I know I put the ring on correctly & was real tight & locked in, but.....Any ideas?? The truck isn't used as 4x4, so no off-roading....