Hello I need help!
Lex IS250, 180k km

(I recommend headphones)
Well, two days ago I got a characteristic ticking-tapping/diesel-like noise.
This occurs only when accelerating(light) in Drive.
There is no sound however, when the car is in Neutral, Park, or running with the windows down.
The sound would disappear as the RPMs are going up, and does not appear on 6th gear at all. I think the car idles well and there are no error codes. Gas mileage in norm.

Considering that this only happens in Drive and can't be reproduced by increasing the RPMs on N or P, what could be the reason?
Tranny? it shifts so well and so far had zero problems with it.