Hope someone can help!

I have recently bought my first Bongo and have a few general technical questions:
1. At about 30-40 mph there is a slight whine from the rear? It goes away at higher or lower speeds - is this in the gearbox / differential and does it need any attention?
2. When the weather was really cold, the Bongo took some starting - often it wouldn't even turn over but it did when putting the autobox in neutral - is this normal? Is it an electrical fault? It works fine now that the weather is warmer.
3. When the engine is cold, the autobox doesn't shift into top / overdrive(?). After 2-3 miles everything is fine.
I am after some advice on these issues
Thanks for reading!