Transmission downshift problem on 99 chevy tahoe

no engine size told of 3
4.8 OR 5.3l or 5.7 GAS???
no transmission told 4L60E
vin code R , T or V? (transmission code) told.
2wd or 4wd? not told
no down shift symptoms told (delayed, or too soon for FLARES)
upshifts ok?????
there is no such thing as a downshift sensor (like muffler bearings?)
the VCM brain that runs the transmission shift points do that.
via the solenoids and VSS tailshaft sensors, and engine rpm.
not scanning the VCM means the 35 errors there are missed
P07xx or P18xx errors, DTC errors.
nobody can fix modern cars lacking a scan tool, or fail we do. hard.

the VCM is crude computer, and has tables in perm. memory that set all shift points and throttle angle too and VSS and engine RPM.
if inputs are wrong VCM will shift wrong called GIGO
The trans with slipping clutch packs will tool. .

I had that sound in my Acura. The transmission was low on oil.
Hope this helps.

It should be near the back of the transmission where the speedo cable used to be.

This happened to me. I had rear end replaced, springs, shocks and universal joints replaced and still same issue. I dropped drive shaft and greased the slip yolk going into transmission. Problem fixed. $4 grease cost me about $1500.
Dave

Is it slipping at all.
If not then your torque converter has failed.
hope this helps FixYa leave feedback

Sounds like the voltage to a the computer is low. verify the Alternator is working puting out 14.5 volts

350 turbo? or 400 thd? makes a big difference i built tranny's for over 40 years

could be modulator

I Do Believe that the Light Assembly Comes OUT Via TORX Screws and The Lights are On a Curciut Panel that is then Removed to Gain Access to Bulbe

The solenoids are located in the transmission. There is one for the 1-2 shift point, one for the 2-3 shift point, and one for the 3-4 shift point. You have to drop the transmission pan to access them.