Why does Holmes get angry when Merryweather bangs his stick in "The Adventure of the Red-Headed League"?

Holmes warns Mr. Merryweather to be quiet so the bank robbers will not hear them. 

Holmes went through a lot of trouble to catch the bank robbers.  He didn’t want Mr. Merryweather to mess it up by creating a racket and warning them that someone was there waiting for them.  Some people just don’t really know how to be sneaky! 

The whole thing starts when Mr. Wilson comes to hire Holmes because he lost his job with the Red-Headed League and he is upset.  Holmes determines that the League was a front to get Wilson out of his shop for a few hours a day to dig a tunnel to the bank next door. 

Holmes arranges with the police to sit at the other end of the tunnel to wait for the robbers.  He knows that John Clay, who was pretending to be Wilson’s assistant, is actually a very intelligent wanted criminal.  Since they buttoned up the league, the robbery will happen soon. 

They wait with the police and the bank chairman of directors, Mr. Merryweather.  Merryweather is kind of impatient and “personally interested in the matter.”  He bangs his stick on the floor and gets scolded by Holmes. 

"I must really ask you to be a little more quiet," said Holmes severely. "You have already imperiled the whole success of our expedition. Might I beg that you would have the goodness to sit down upon one of those boxes, and not to interfere?"

The plan works, and Mr. Merryweather thanks Holmes profusely.  The bank is not robbed, and a very dangerous criminal is caught.  Mr. Wilson lost his cushy job with the Red-Headed League, but at least he solved the mystery of what happened to it.  I guess the old saying is correct.  If something seems too good to be true, then it really is!