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Wining From Underneth - Rear End or Tcase?

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2000 Silverado, 5.3 - 247K miles.    Since I have purchased this truck 6 months ago, it makes a wining sound from underneath when driving. This sound only happens when I let off the gas.. and coasting.  First thought is the rear end, or the Tcase.   I did change the fluids when I got the truck - rear end fluid looked fine, but I changed to Royal Purple.  Tcase barley had any fluid,  what fluid did come out was gear oil...not ATF like that case calls for (NP 261 with manual shiftier)  Anyway, I refilled with syn ATF....all good.  Also replaced the Tcase half recently because it got a hole - pump rub issue. 

 

I'm thinking I should refil the Tcase with gear oil?  bad idea?  I know many folks do this to quiet their Tcase even though the case calls for ATF.  Seems like the only risk is the seal to transmission going bad and letting gear oil in the trans. 

side note - u joints are new. drive shaft has been changed at point (not OEM)   truck drives real smooth, no shakes or anything. 

Truck is just used around the farm... about 4k miles per year. Not a big issue either way. 

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