Clicking noise from drivetrain under load

So when I pedal under high pressure, one crank arm loudly clicks one time at position 1 or 2 (non-repeatable click) and when I continue, the other arm does the exact thing at the same position.
The click can only be produced once in a given arm before the other arm clicks under pressure at the said position, so the click alternates between the arms under pressure, once each.
The same thing happens when the bicycle is standing still while putting pressure on the pedals (holding the brake).

I hope my explanation is clear; it seems a bit wordy.

It is not the pedals or the crank arms themselves that are the culprit, coz I made sure they are properly tight.
I am using a sealed bottom bracket and I suspect it is the source of this.

What do you think is the cause? And if it’s the BB, do I have to replace it?

Thanks