Transmission fluid is leaking into the engine
I presume you have an automatic transmission. Is there transmission fluid in the coolant? If so, probably a leak in the transmission cooler tube inside the radiator. That’s a serious problem b/c the leak can go the other way too, engine coolant will go into the transmission and result in some seriously expensive damage. Suggest to not defer getting this addressed by your shop.
the vacuum modulator on the transmission uses a vacuum line to the engine & when the modulator diaphram ruptures it will suck transmission fluid from the transmission into the engine. Check the vacuum line, if it has transmission fluid in it, replace the modulator.