These irregularities are caused by. You should be able to find the stud using a magnet tape measure and a nail and hammer.
Studsensor A150
How to find a stud with a stud finder. Look for a column of bumps or dimples in the surface. You move the magnet across your ceiling until the magnet reacts. The baseboard is nailed to studs and outlets are usually. The stud finder will then indicate if it is calibrated or if you need to move it to a different area to calibrate again. Examine the baseboard for nails or outlets. You can also try holding a flashlight against a wall and looking down the wall to see if you can tell where two pieces of drywall meet.
Or you have to measure 24 inches to find the stud. If you start in a corner and measure out 16 inches and you dont find a stud you should find one at 24 inches. Recalibrate the stud finder if necessary. When the stud finder alerts you to a studvia a beep or flashing light depending on the finder in questionretrace the last few inches with the unit to double check where the alert is occurring. Mark the location on painters tape. Usually you can do this by positioning the device on the wall where there is no stud then activating it.
Some stud finders have to be calibrated. Shine a flashlight at a steep angle against the wall. When a home is framed the wall studs are usually spaced 16 or 24 inches apart. Hold the stud finder against the wall and move it along the surface at the installation height of the fastener. Joist holders nails and other key ceiling elements are made from metal. An edge finding model signals when it senses the end of open space behind the wall and the edge of a dense object such as a stud.
You keep poking holes in your ceiling until you find the stud. Holding a stud finder against the wall comes next. How to find a wall stud without a stud finder.