LU track roar in tube tunnels (i.e. not SSLs) probably breaks H&S limits for sound levels.
I'm not convinced there's much "rail roar" on LU nowadays, in the traditionally understood sense at least.
On LU there a *lot* of rail grinding activity nowadays, the Victoria Line in particular almost sings to it. That line always was very bad for noisy rails (presumably due to the higher than average speeds), but the regular grinding seems to have reduced this, albeit replacing one set of noises with another!
Elsewhere, there's been a significant increase in noise on certain very specific sections of line, mainly curved sections. My understanding is this has been due to a change in the track form associated with the Night Tube, reducing vibration but at the cost of increasing noise. There's certainly a massive number of complaints coming in on a daily basis about this, and presumably solutions are being sought - clearly it's not a straightforward issue otherwise a solution would have happened by now, as this has been ongoing since the Night Tube came in.
Then there's the traditional rumble experienced on certain sections of line, mainly those with bull-head rail and a specific type of track form where the sleepers are concreted in with no ballast at all. There's still a few sections like this, albeit declining. Again, I think the noise comes from the track form as a whole rather than just the wheel interfacing with the rail.
Must admit of the sections I regularly traverse it's hard to readily think of *any* sections where there's roar of the traditional sort. The Terminal 4 loop (Hatton Cross to T123 via T4) used to have a very distinctive tune to it, been a little while since I last went round there so may have changed now. If it's still there then this is probably the best to be found on LU, especially if the driver goes round at full speed.
Thinking of the original question, on the mainline it certainly was the case that some of the sections of the slow lines between King's Cross and Peterborough were quite loud, generally those sections retaining wooden sleepers. There was a particularly loud stretch on the down slow around Welham Green. Nowadays virtually all my trips tend to be on the fast lines as I adjust my travels to use 365s as far as possible, so I can't readily say what the current situation is. I'd tentatively suggest Hitchin to Huntingdon on the slow lines might still be a good section, especially on a service which skips some of the intermediate stations (most of the evening peak KX-Peterborough GN fast services run on the down slow north of Woolmer Green).