Truck having a hard time cranking battery is good alternator is good

What is your question?

Truck having a hard time cranking the number I hold the inition down need truck I will eventually crank but it sounds like the battery is about to die when it does once it cranks it's fine as long as I don't leave it off more than 15-20 minutes if I leave it off more than 20 minutes it does the same thing again having a hard time cranking like the battery is dead but I've had to battery and alternator check there was a ticking sound on the passenger side I changed the cam sensors about a month ago on both sides there was a tapping sound on the passenger side I thought it was the belt tensioner but then it started to get worse I talked to a mechanic he told me to put some cleaner in the oil run it for a couple days and then change the oil never stop the tapping which it did but it still hard cranking thank you for your help

What makes this problem better or worse?

When I go and get in the truck and turn the put the key in and turn the ignition and struggles hard to crank I mean really struggle to crank back the battery is about to die but once it cranks you can cut it out crank it up cuz I'll Crank It Up and it runs fine but after it sits 15 20 minutes it starts it again

How long has your 2006 Lincoln Mark LT had this problem?

Just started 3 , 4 days ago