Most all of them do that (they are 35 years or older). Start by changing the filters and making sure the oil level is ok, it could have some internal leaks but if if works ok after the oil warms up I would not worry about it unless there are other problems.
I have a 2440 with the same problem. There is a charge pump in the transmission that feeds the main hydraulic pump. This charge pump is probably getting bad so it takes a bit until the main pump gets oil.