Deep ‘thrumming’ vibe from chain set when in top gear?

[Sloth]

Is the limit screw set properly as it's the smallest sprocket ?

It was set up at the shop so I can only assume it’s ok.
Not something I know how to do, I don’t mess with anything like that.

 

[Sloth]

This 'thrumming ' used to happen to me when fitting a new chain, it usually lasted a few rides then passed. I've had it 3 or 4 times, always when fitting a new chain.
The logic I applied ?, perhaps incorrectly, new chain, old cassette teeth caused a temporary mismatch and vibration. This would pass as the two meshed with a little time and use.

So perhaps because your smallest sprocket has rarely ever been used, similar is occurring ?

Perhaps, the whole GRX set up is brand new with less that 100 miles on.

 

[Ajax Bay]

Fossy: Is the limit screw set properly as it's the smallest sprocket ?

It was set up at the shop so I can only assume it’s ok.
Not something I know how to do, I don’t mess with anything like that

Well do you want to solve this or not? The 'thrumming sound is not "OK" is it?
Assuming mechanical RD, unscrew the little screw marked 'H' a quarter turn before your next ride; and check.

 

[Sloth]

Well do you want to solve this or not? The 'thrumming sound is not "OK" is it?
Assuming mechanical RD, unscrew the little screw marked 'H' a quarter turn before your next ride; and check.

Wow, no need for that tone!
I don’t really know what that limit screw is or exactly how it works so why would I mess with it?
I’ll read up on it I think and appreciate your advice.

 

[roubaixtuesday]

Not something I know how to do, I don’t mess with anything like that

TBH, if a single screw adjustment isn't fit you, there's not much purpose in posting her for advice. Any advice will require some mechanical adjustment. Better to just take it to the shop.

 

[Ajax Bay]

if a single screw adjustment isn't for you, there's not much purpose in posting here for advice. . . . Better to just take it to the shop.

Your tone is so much more suitable, though aiui the OP has sought LBS advice and they said:
"it was ‘normal’ and even if they fitted a new free hub it would ‘be the same after a short while so why bother’" in emollient tones.

I don’t really know what that limit screw is or exactly how it works so why would I mess with it?

Because you wish to make an effort to stop the 'thrumming'.
https://www.manualslib.com/manual/2299949/Shimano-Grx-Rd-Rx810.html?page=11#manual
"Adjusting the stroke on the high limit" half way down manual

You're welcome.

 

[Yellow Saddle]

I have had a GRX 1x11 fitted in place of the 105 2x that the bike came with.
The bikes a Giant Fastroad Advanced 1 (2022).
We all know the annoying clicking of chain rub when in the largest rear gear and at one extreme of chain angle.
We’ll, imaging the other extreme when in the smallest rear gear.
Instead of a clicking or typical chain rub sound, I get a very low, thrumming sensation at the opposite end of the sound spectrum to typical chain rub.
It only happens when I’m in the smallest gear (remember it’s a 1x)
It’s also almost like I feel some drag or resistance when pedalling in that gear, as if the front chain ring is somehow not aligned and causing this weird low thrumming vibration with every pedal stroke, but only in the highest/smallest rear gear.
It’s not ruining my rides but it just doesn’t feel ‘right’.

I gather from your description that you are describing a vibration of sort and I'll guess that the freqency of that vibration is in line is X/R where X is the number of teeth on the front sprocket and R is 1 revolution of the crank. In other words, each time a new tooth enters the advancing chain. If you're a figures geek you can measure your revolutions per minute and work out what the frequency of the vibration is by comparing it to something that youre familiar with - a 50Hz AC voltage hum on a transformer, for instance. But, I digress...

The vibration is caused by an incoming tooth (cog) hitting the chain roller and then forcing itself into the space between rollers. There could be two reasons for that:
1) The chain is brand new. Remember, a new chain is sligthly shorter than spec because assembly riveting leaves a chain imperfect and it needs time to be pulled into alignment and perfect new-chain pitch of 1/2"

2) You ride a singlespeed and the chain has elongated to beyond spec, and the incoming tooth now also hits the roller of the chain. This is only possible on a singlespeed/hub gear with tension in the bottom run of the chain. On a derailer bike the jockey spring will prevent such a chain to work at all - the famous chain skating scenario.

The vibration mechanism in 1 and 2 above is similar, except for in one the incoming cog hits the back roller and on the other, it hits the front roller (chain alternately to short and too long).

The vibration will go away soon. Just ride.

It happens more in the small rear sprocket (I assume a 11tooth) because the way a chain link rides over a small sprocket makes it see-saw at it crests and that see-sawing motion makes the chain ever so slightly longer and shorter. To visualise this, imagine a 2-tooth sprocket in rotation with a chain. You can see the chain hop up and down as it crests.

 

[Sloth]

Thanks, I will be sure to update after I’ve ridden it a while longer.
I did check the RC again yesterday and there is some slight play if I pull and push it (the cassette) away and back toward the hub. Not much but definitely noticeable.
Would that not be more likely to move on the smallest sprocket as it’s smaller and furthest away from the wheel hub so has more force pulling it away from the hub?