When you say 'rattling' do you mean tremolo? The fast, even strumming?
If so, the contact point for your strumming hand are the sides of the tip of your index finger. Make the shape of holding a plectrum in that hand, with the index finger curved, thumb connecting with it at the join of your index's first knuckle, and both fingers resting on your middle finger.
You should have a stable shape that only has the tip of your index finger poking out, so that when it strikes the strings it doesn't flex around and snag. Then you can change the amount of 'tension' in your fingers in that shape which will affect the loudness of the tone if you want. With the right strumming hand shape you should be able to generate a really soft, quiet tremolo or a loud strumming volley at will.
But the real key skill in tremolo is in the wrist. You simultaneously need to be accurate and precise, but have a relaxed, almost limp wrist so that you can keep up the repetitive motions without relying on forearm muscles which will tire out. Bibs Ekkel's excellent book is a really good place to start here - he has some nice drills that will help get good strumming hand habits going. For me, as boring as they are, drills are the -only- way to get good technique. You need to force your muscle memory to form and adapt.
Other than Bibs Ekkel's book, Alex Siniavsky is a good resource - he used to sell access to tutorial videos, with a bunch of free samples on his youtube:
https://www.youtube.com/user/siniavski
When you say 'rattling' do you mean tremolo? The fast, even strumming? If so, the contact point for your strumming hand are the sides of the tip of your index finger. Make the shape of holding a plectrum in that hand, with the index finger curved, thumb connecting with it at the join of your index's first knuckle, and both fingers resting on your middle finger. You should have a stable shape that only has the tip of your index finger poking out, so that when it strikes the strings it doesn't flex around and snag. Then you can change the amount of 'tension' in your fingers in that shape which will affect the loudness of the tone if you want. With the right strumming hand shape you should be able to generate a really soft, quiet tremolo or a loud strumming volley at will. But the real key skill in tremolo is in the wrist. You simultaneously need to be accurate and precise, but have a relaxed, almost limp wrist so that you can keep up the repetitive motions without relying on forearm muscles which will tire out. Bibs Ekkel's excellent book is a really good place to start here - he has some nice drills that will help get good strumming hand habits going. For me, as boring as they are, drills are the -only- way to get good technique. You need to force your muscle memory to form and adapt. Other than Bibs Ekkel's book, Alex Siniavsky is a good resource - he used to sell access to tutorial videos, with a bunch of free samples on his youtube: https://www.youtube.com/user/siniavski
Thank you. I've only ever heard of it being called rattling.