Sand is insidious. It gets everywhere, and once in, it gets comfortable and doesn't come out.
Even though you say you cleaned the chain, there may still be sand on it on glued to the chainring, but the BB isn't immune either.
I'd start by slipping the chain off the chainrings and draping it on the BB shell holding it so it doesn't touch the rings at all. Then spin the crank, and feel and listen. If there's not a hint of grinding or crunching, odds are the BB is sand free, and you can try cleaning the chain again.
Remebler there are three areas where sand can hide, the chain, the cassette an the chainrings. Sand in any one of these will be picked up and transferred to others in fairly short order, so doing one, but not all three is not going to get the job done.
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