Transmission oil pump

Upon the vehicle cranking with no problem I hear what I think is the transmission oil pump making a whining noise. If it is making the noise I can put in gear and ride. However I may get a mile or five miles down the road and I roll to a stop. I can get out and tap on the transmission pump a few times get back in crank and ride again for another mile maybe five to repeat same process. Can someone please guide me in the right direction as to what could be the cause? Am I going to need a new transmission completely? Any help would greatly be appreciated. My mom and I only have this way of going and my dad just passed away last month so it is up to me to take care of and fix things myself now. Thank you in advance for any help.