"noticed a dip" is correct and usable in written English.
You can use it to describe someone noticing a drop or decrease in something, such as numbers, income, energy level, etc. For example, "He noticed a dip in sales this month."
Ryan said he had not noticed a dip in effort or focus in practice.
This time, he said, he noticed a dip in enthusiasm: "At that show last year, kids showed up wearing heels.
MARKET OVERVIEW "We've noticed a dip in the demand for downtown condos," said Andrew Carros, a real estate agent with Sotheby's International Realty Canada's Vancouver office, "but it's a slight correction rather than a downturn".
However, Trump said he has noticed a dip in business since the 2016 election.
City workers were working on repairs when they noticed a dip in the pavement, according to the Detroit News.
This year, though, retailers noticed a dip after Caterpillar began to warn employees that payments for 2013 could drop.
"Well," he told the president, "we did notice a dip in demand at that point in time".
James Purnell, the BBC's strategy director, has said the "sword of Damocles" is hanging over BBC funding with cuts to one of its biggest shows, EastEnders, reversed after viewers began to notice a dip in quality.
We have noticed a perceptible dip in forward fares".
There the story might have progressed along a predictable continuum had Dr. Drager's parents, Marvin and Lenore Drager, not noticed a slight dip in her grades.
Though Juicy Couture noticed a small dip in same-store sales in September, Lucky Brand recorded a 24 percent increase; sales at kate spade, which caters to a wealthier demographic, more than doubled, gaining 114 percent.
