mazdas b series motors B6 1.6 dohc, and the bp 1.8 dohc, have crazy valve lash. Oe rthe years, but i noticed it more when the car is low on oil. And or the car has sat for along time, or in cold weather.

but you can help it by running some sea foam or gum out on the valves, or take the valve cover off, and get a tooth brush, and go at it head to head, with some gum out.. and flush it...

but it could be a shitty gasket... since it just start happening.