My CX5 has what sounds like a slight knock on cold starts. It is a 2015 with 90,000 miles. I just noticed it last fall when I started the car and got out to do something while it ran. After the initial fast start when the engine slowed so did the knock. It does not come back the rest of the day. Since I do not notice it if I am in the car the entire time I do not know how long this has happened. I just did my annual drive to Florida and back in January with no issues at all.

 

Welcome to the forum

 

You probably should have noticed this all along.
The Mazda Skyactive engine has variable valve timing and "can" be rather noisy on a cold start-up compared with other engines.
If you have never noticed this before you may have other issues.

 

General information: Balance shat assemblies on some engines with higher mileage will start to make more noise than normal during cold starts until the engine is up to normal operation temperatures seen as water temperature at 180-195F.
To find o the noise locations you can use Audio hearing tools or equipment to pinpoint where the exact sounds are being generated. That combined with a handheld temperature meter you can find the noise locations.