Knocking around bb

I just bought a new bottom bracket a few weeks
Why do i hear knocking around my bottom bracket
After a long ride and how do i fix it

Random as it may seem… I had a new bottom bracket installed a few months ago and I had a knocking coming from, what I thought, was that area… it turns out it was me seatpost that wasn’t secured properly

When you say knocking is it something you feel through the pedals when pedalling? Do you feel it when not pedalling?

I feel it

I’d grab both pedals and try and firmly move them both laterally and vertically and see if you can feel any movement. BB may be loose, cranks may be loose. Be prepared that it’s something else.

Ive got a horrible knocking coming from what seems like the BB area but it only manifests at a certain cadence/resistance (its silent at low resistance at any cadence or at low cadence when the resistance is high) and the BB/cranks/pedals seem solid of the bike. I could be wrong but I suspect that its the pedal/cleat interface.

Not had a problem with screw thread bbs but certainly had problems with press fit bbs - I had a knock after fitting a new set of bearings (bb30A) when out of saddle or pushing on - so had another set fitted and all ok. Don’t know exactly why the set of bearing made a knock noise (poor quality, not fitted when? Who knows).

Also, see if there are any witness marks anywhere from something rubbering and also check that the drive side crank arm isn’t hitting the front mech. Lots of spacers being fitting now too (Cannondale topstone for example ). If a shop fitting the bb - take it back.

Lots of reasons.

Incorrect seating
Improper cleaning of bb area
Wrong grease/no grease on cups/seating area
Bb on bike is misaligned/oval/too big/too small
Threaded too tight/too loose
Bearings shit out of the box (it happens)
Crank not installed correctly (not pushed in all the way)

And as always, it may not be your BB at all. Was that the only change to the bike?

Double-check that your pedals are properly torqued.

101 possible reasons and damn hard, potentially, to diagnose IRL, much less over the net. Tbh if you’re not a reasonably accomplished home mechanic, this is one for the LBS.