Indoor unit noise and min pipe length

I have recently had a Fujitsu multi split system installed with 3 heads. One head is back to back to the outdoor unit with approximately 3m (or even less) of pipe work. According to spec of this specific multi split, the min pipe length for each unit needs to be 5m. Info here and there across the internet seem to suggest that this is what's causing the "whoosing" sound I hear in between fan operation. This sound is akin to a toilet cistern filling up after flushing. The other sound I hear is a slight constant humming sound, but the whooshing sound is definitely louder.

Fujitsu thinks it's the short pipe length causing the noise and the installer has agreed to extend the pipe length in a week or two.

My questions to this group is...

Has anyone here experienced the same, and was the short pipe length definitely the cause?

What is the right way to extend the pipe? Supply a new pipe with a 5m length, or weld an additional 2m? I suspect that my installer will do what is easiest/cheapest for them and weld an additional 2m and loop it behind the outdoor unit.

Will welding a pipe be an issue long term? Are they particularly susceptible to leaks?