Noise when pedalling uphill? I have a bike that recently began riding again after many years. 

It is a flip-flop hub. When on the fixie side, there is no noise. When on the freewheel side, there is a grind when riding uphill.

I greased it, but made no difference. What can I do?

Is it better to have the chain tight or slightly loose?
I get that too. My bike is about three weeks old so it has Nil to do with  bike age. I remedied by using teflon chain wax / lube. May pay to see if it's the pedals or the chain / derailers are all in good nick. Sometimes when a bike has not been ridden for sometime it needs a service or at least a inspection just like a car.