As someone who's been to Fratton Park a few times (Cov games and then spare tickets from mates/business invites), I'd say they're noise levels can be some of the best in the league. I agree with a comment before about their noise levels last season when they spanked us 4-1 - it was proper loud from them that day.
And you ask fans of any Championship social media page who the 'best and loudest' fans are, I'd say the majority always say Pompey.
That said, the CBS is great when it really gets going. I think the weather definitely plays a big part. When it's dull and wet, people are naturally less inclined to get involved.
In terms of away fans, I'd say we're definitely the loudest and those 'best and loudest' comments on social media, often has us as the best travelling support.
I'm begrudgingly taking my lad to Fratton Park this coming weekend with his pompey supporting grandad. Hopefully it'll be the last time he wants to go. The one thing that is sh!t about Pompey fans is their lack of songs. Other than PUP and Blue Army, they've got nothing of substance.