Transmission Rapidly Shifting Between 1st/2nd When Cold

[Jon87V20]

As of the past few days, when the truck is cold it's rapidly shifting back and forth between 1st and 2nd when I'm just giving it a tiny bit of throttle leaving my neighborhood. It's not really doing this at higher speeds (30mph or more). It's also shifting less rapidly but still noticeable between 2nd and 3rd when just coasting (engine speed is almost the same as it would be idling) with no throttle input.

I checked the fluid level about a month ago and everything looked okay. I keep forgetting to check it again now, I'll have to look at it again tomorrow.

Is this something I should be concerned about? If the level is normal could this mean I need a filter change?

The truck is an '87 Suburban V20 with a 350 and what I believe is a TH400.

Thanks!

[Jon87V20]

Update: I checked the fluid yesterday and it still reads normal.

[Jon87V20]

I've also noticed, when it's cold there's a whining sound coming from the transmission that happens in all gears except reverse. The only way I can explain the noise is, it sounds very similar to a power-steering pump that's low on fluid, or a supercharger sound. It'll make this noise for about five minutes and once it's getting warm the shifting problem and the noise seem to go away. Does anyone have any ideas on what this is? I'm just trying to figure out if this means there's an imminent failure on the horizon, or if this could be something else like the governor or the modulator or anything else I can do something about. This is my daily driver so I'm hoping I won't need to replace the tranny any time soon.

Thanks!

[gwcrim]

The whining noise could be the pump.  How many miles on the trans?

[Jon87V20]

gwcrim, not sure on the milage. It died a few days ago. It start making more noises. It sounded like there was metal pieces being slapped around inside the pan when it was in reverse. Later on that day it couldn't get up to third gear unless you let off the throttle. By the time I got home it was slipping very badly. Lucky I made it back and got it in the garage.

[Jon87V20]

So after taking a look at it, I'm not even sure what this transmission is. Can anyone tell by looking at this picture or do I need to get a picture of a different part of it?

EDIT: After crawling under there further, I could see there was another curve in the pan on the corner on the other side of the pan and it says Hydromatic on the bottom. So I'm assuming it's safe to say this is indeed a TH400 correct?

Also, does anyone know if a TH350 out of a '77 Corvette would be compatible with this truck? My dad has an extra one and I don't know when I'll have a $1,000+ to spend on getting this back on the road. Thanks!

[bd]

Transmission Identification

THM 400

[st3vomettler]

It's a th350 because it hose one corner missing

[Irish_Alley]

It's a th350 because it hose one corner missing

the th350 does have a corner missing but its not curved.

[st3vomettler]

I see the rest of it hidden under the exhaust pipe