USALLY it's because the Transmission cooler that's located inside the RADIATOR HAS DEVELOPED A LEAK.The COOLING system & TRANSMISSION need 2 b FLUSHED after the RADIATOR is REPLACED -- OR BYPASS the RADIATOR TRANS cooler & put on an EXTERNAL cooler INFRONT of the existing Radiator.
It sounds like the head gasket is leaking.
I just had the same problem. It started with noticing just a thin layer of light brown foam when I removed the radiator cap. There was no foam or anything else wrong with the engine oil or trans fluid. My mechanic took a quick look at it and said to just keep an eye on it. I checked it every week with no change noticed, then sort of forgot about it for a month. When it started to shift a little rougher than usual, I checked the trans fluid again. This time the fluid was a nasty color of rusty brown and very opaque. Apparently it is not that uncommon to get a leak between the radiator coolant and either the engine oil or the trans fluid. Just speaking from my own experience, I believe it is more common to have the engine oil leak (bad head gasket or cracked head) in which case there will be obvious white/brown foam in the engine (on the oil cap) when it is checked. The trans fluid is under higher pressure than the radiator coolant so most of the flow is from the trans to the radiator, but as the system cools down, it can (and does) flow the other way. It just takes a little longer to notice the problem. Since radiator fluid can destroy an automatic transmission and trans fluid can destroy the seals in the radiator system it is very important to figure out what�s going on as quickly as possible...and NOT �just keep an eye on it�....won�t be going back to that shop. My van (a 94 Pontiac Transport) is in the shop now getting a new radiator (has two internal trans coolers inside of the radiator), and when that�s done I�m driving it directly to have all the trans fluid flushed. With any luck, it will start to shift smoother ...if not, it will have been a VERY expensive lesson. Hope this helps.
From the transmission? No? But antifreeze & transmission fluid can mix. Your transmission uses a small portion of the radiator as a transmission cooler, normally at the very bottom or side. In very rare instances, this cooler can develop a leak and allow transmission fluid and antifreeze to mix together. The only fix is to replace the radiator.
This is a fun one. Pop the hood. Okay, now you're facing the engine...you'll see there's a hose to your left coming out of the top of the radiator, look under that hose and you can see the transmission...the dipstick screws right into the transmission...make sure you're checking it with the engine OFF, and also be careful not to burn yourself on that radiator hose, it can get really hot. The dipstick is quite aways down and rather short.
If your car has an automatic transmission, there will be a short dipstick near the front (radiator side) of the transmission. With the transmission warmed up (go for a short drive), park on a level surface, and with the transmission in park or neutral and engine running, pull the dipstick out and check the fluid level against the markings on the dipstick.
The transmission has a very small dipstick that is yellow located between the transmission mount and transmission on the side closest to the radiator. There is a 13mm bolt that holds the dipstick in place. Do not remove any bolts from the top of the transmission or you can end up having to have the transmission fixed.
If it is motor oil in the anti freeze, then it is either a cracked head, cracked engine block, or a head gasket blown. If it is transmission fluid in your antifreeze, then the radiator is leaking from the transmission cooler ----- although usually that would cause antifreeze to be in your transmission fluid.