Baby grunting at night

My son goes down to bed very well initially. Typically he’ll have his last feeding around 9pm and then sleep until 2-2:30am, which is great. But then after the 2am feeding, he doesn’t go down as easy. He’ll fall asleep fine, but after laying him down in the crib next to my bed, he’ll begin grunting within 20-30 minutes, and he’ll continue to grunt for an hour or two off and on. I’ll usually have to get him up about 3:30 or 4am because he just keeps us awake with his noises. This is the only time of day/night he does this. I don’t know why! It’s completely exhausting me!

Does anyone have experience with this? What could it be? I thought maybe it was gas, so I give him an extra 15-20 minutes of burping and bouncing after that 2am feeding, and I do bicycles as well. But the grunting keeps happening. It’s like he’s asleep but still making the noise!