I have never heard this phrase before, so I don't think it's an idiom. You can't have a herd of grain - ask the person what they actually said.
To take something that someone says with a grain of salt means that you should not necessarily believe everything he/she tells you.
This is not an idiom. A herd is a group of certain animals, like sheep and cows. This phrase just means a group of sheep. You might have heard someone comparing people to a herd of sheep. That is not an idiom, but a simile saying that some people act like sheep.
Noisy, unsubtle, obvious. Something which is impossible to overlook. Example: "I have a two year old and a four year old, and they"d put a herd of elephants out of work."
It's the herd mentality...everyone else is doing it, I will too.
=What the does grain mean?=
It's not really an idiom. It means "what are you thinking about."
It is not an idiom, it means your nose is itching.
RFP is not an idiom. It's an abbreviation.
Herd is a group of animals that stay together.
"Sieve" is not an idiom. See the related link.
It's not an idiom. It means the tip of your nostril.
idiom means expression like a page in a book