"noticeably weaker" is correct and usable in written English.
For example, you might write: "The company's performance in the second quarter was noticeably weaker than the first."
I was noticeably weaker".
In the industrial areas, protections have become noticeably weaker.
But if your grip has gotten noticeably weaker, talk with your doctor.
Compared to the iPhone 6S the screen is noticeably weaker, with poorer blacks and vibrancy.
And this market, as demonstrated by the Nasdaq composite index, has been noticeably weaker in recent months than other broad measures of stock prices.
If Hamid is at the top of its game on weekdays, it is noticeably weaker on Saturday nights after sundown when the Jewish Sabbath ends.
The tourism-driven sector known as leisure and hospitality also was "noticeably weak" last month, he said.
The first ratings returns for the network were noticeably weak, though the picture has brightened more recently.
Everything being said is vital, if noticeably weak in confronting the environmental risks of processes like hydraulic fracturing.
Microsoft's recent purchase of a stake in Facebook made its ambitions clear - and Google has been noticeably weak on this front.
However, in our case, the (101) peak presents the highest diffraction intensity, whereas the (110) peak intensity is noticeably weak.
