“But um…” sounds like the sting of a drummer (“ba dum tiss”). A joke was posted on Reddit—Dadjokes on March 13, 2020:
       
“I don’t usually brag about my drum jokes, but um…tss.”
     
“Two drums and a cymbal fall off a cliff. Ba-Dum-Tss” is a related joke.
       
         
Wikipedia: Sting (percussion)
A sting is a short sequence played by a drummer to punctuate a joke, especially an obvious one. A sting is often used as accompaniment during cabaret- and circus-style shows.
 
Sometimes the sound of the sting is written ba dum tsh, ba-dum ching, ba dum tiss and occasionally ba dum tis. In British English, boom boom is used, for example in “Ha ha ha! Boom! Boom!”, the catchphrase of the children’s television character, Basil Brush.
   
Twitter
tim
@TimothyTheFirst
if you say “but um…” really, really fast, it sounds like a drum roll after a while.
1:12 AM · Jul 2, 2011·Twitter Web Client
   
Twitter
SMFC
@Corey814
People who say “but, um..” you know you sound like a drum? Also a little stupid..
4:05 PM · Nov 13, 2012·Favstar.FM
   
Twitter
Moira
@moirakocain
It really bugs me when people say “but um” and it sounds like “baddum”... It makes me think of a drum.
7:22 PM · Jan 10, 2013·Twitter for iPhone
   
Twitter
Luke ☭
@Wazzok1
But um in an American accent sounds like
a drum beat.
“BUT UM TSSS”
8:18 AM · Aug 25, 2013·Twitter Web Client
   
Twitter
☻
@lavenderxlarrie
“but um” a drum going budumchhh
5:45 PM · Jun 23, 2016·Twitter for iPhone
   
Twitter
I dont have feelings these tears are allergies ok?
@annikarainbow
I say “but um” a lot so I want to hire someone to follow me around and say “tss” every time I say “but um” so that I can finally be a drum
1:49 AM · Jul 6, 2017·Twitter Web Client
   
Twitter
James Coster
@jjcoster
@DylanDreyerNBC
A friend used to say “But, um…” b4 everything he said. So we’d cut him of by saying “But-um-bum!” (like a drum roll) Ha!
9:16 AM · Feb 8, 2019·Twitter for Android
 
Reddit—Dadjokes
Posted by u/demand404 March 13, 2020
I don’t usually brag about my drum jokes but um…
tss
 
Twitter
Monsieur_Baguette
@Mr_Baguette_
I don’t usually brag about my drum jokes but um…
tss
7:23 PM · Mar 13, 2020·Twitter Web App