*Briefly* disable all other plugins and enable the default Twenty Twenty theme. Please also disable all add-ons. If this fixes the problem, as it probably will, enable everything one by one, checking each time to see when the problem returns.
Thank you so much. Yes it seems the problem is in the plugin Autoptimize. After deactivate it so the string now gone.
As much possible we need plugin like autoptimize, could you suggest some plugin like this?
