Foul tasting raspberries

Hi,
Can anyone help me with this please?

I have a row of summer fruiting raspberries, they are currently fruiting well and I have covered them to protect them from the birds.

I picked several under ripe berries to make jam and everything was fine.

Now that I am waiting for them to ripen and eat fresh or put in deserts one in six or seven tastes absolutely disgusting and there is no alternative but to spit them out. It's a sort of musty metallic taste.

The berries themselves look perfect, with no sign of raspberry beetle damage. I occasionally see the odd ant on them or some tiny little flies.

I've tried googling and found nothing, any ideas?

Thanks

Thanks, they are ripe and look perfect 

Hmmm, I don't think so, and why only one in six ? You'd think a whole bunch together would be off. Today I tried 6 or 7 and all were lovely. I thought I'd ridden my luck enough and called it quits  Yesterday I gave my neighbour free range with a warning and she came across 3 or 4 spit out worthy berries from a small random selection.

It's a real head scratcher and has got us all puzzled.

I'm wondering if the comfrey feed I make might be the cause but then it hasn't affected the strawberries and other fruits so it can be that.  Dunno?

It's the odd one on every plant, well along the row, not sure if its every single plant they've all sort of merged into one over the years (3).