First, the noise you are hearing is likely the pre-flash/focus assist feature mentioned in an earlier response. That occurs in lower light conditions and might make a noise similar to what you described.

Easiest solution is to not use the built-in flash. They are very wimpy and under-powered, located in the worst possible place for redeye and ugly shadow problems, put a heavy drain on the camera's batteries reducing the number of shots per charge dramatically, and are pretty slow to recycle. If I could, I'd buy a camera without a built-in flash! Instead buy an auxiliary flash and solve all your problems! Even the smallest and weakest auxiliary flash is a huge improvement over the built-in used in any camera.

External flashes use a different method of focus assist, too. They noiselessly project a "red grid" for the camera to focus upon, instead of rapidly (and obnoxiously) "strobing" white light the way the built-in flash do.

When shooting in quiet conditions, you also may hear some noise from the Image Stabilization of the lens working. That's easy to check... just turn it off at the switch on the lens and see if the noise goes away.

Micro motor lenses, in particular, make some noise when focusing, too. USM lenses are quieter (and a whole lot faster focusing), but not entirely silent. STM lenses are the quietest, faster focusing than micro motor, but not as fast as USM.

The Sony HC300 bridge camera uses a tiny 1/2.3" CCD sensor to capture 20MP images... while the T7i uses a more than ten times larger APS-C CMOS to make 24MP images.

Try this.... put both cameras in Program mode and set them to ISO 6400 or 12800, then take a couple shots in lower light conditions (depending upon the type of lighting, you might want to set a Custom White Balance in each camera). There is NO WAY that the HC300 with that tiny, super crowded CCD sensor can come anywhere close to higher ISO capabilities and image quality of a much larger APS-C CMOS sensor like the T7i uses. If you are still seeing superior images out of the HC300... most likely you are doing something wrong. IMO, 95 out of 100 "camera problems" are actually "user errors"... no fault of the gear at all. However, I never rule it out... there may actually be some fault with the Canon camera.

Are you shooting JPEGs? (Well, do'h! Yes, sure you are with the T7i.... in full "Auto" it won't let you shoot RAW.) Have you set it to smaller, lower quality JPEGs?

T7i uses a 45-point phase detection AF array in the viewfinder or Canon's Dual Pixel Auto Focus in Live View (phase detection too, though it uses pairs of pixels embedded in the image sensor, instead of the array seen in the viewfinder). Either of those is bound to be faster than the contrast detection AF the HC300 uses. I would bet the T7i also is able to autofocus in much lower light conditions. It's rated to -3EV using the viewfinder focus array... That's approx. "moonlight". I can't find any info about the HC300's low light focusing ability... but typical of that type of camera is -1EV at best... but often -0.5EV or less.

Next, download and compare images on your computer screen. Don't rely on the image review on the camera's rear LCD screen. Those screens are not calibrated and are strongly influenced by ambient light conditions. Also the T7i's screen is 1 million+ pixel, which will be more "critical" of images in playback, than the HC300's 460,000 pixel screen.

Do you always just shoot in full Auto mode? If so, maybe a DSLR isn't the right camera for you. I don't mean that in a condescending way.... just as a matter of practicality. Much of the point of a DSLR is to give the photographer a lot more control ... but not everyone wants or needs that in a camera.

If you like to use other modes, maybe you should use those for comparison purposes. At the least, you probably should give the T7i more than just a brief workout, before deciding whether or not it's the right camera for you (unless, of course, there's actually some fault with it).

Those crackling sounds are preflashes and are normal. Here is an explanation I found on DPreview.

"Question: I've noticed when using the built-in flash when I press the button halfway to focus the flash sputters numerous times - sounds like a sparkler going off and at the same time the flash emits light for each crackle of sound. What is the camera doing at this point? I've haven't come across anything in the manual yet that mentions this.

Answer: Those are preflashes to aid the camera with autofocus in low light. Its to illuminate the scene a bit so that AF sensor can pick up an area of contrast. If you look again in the manual with this information, you will find it there."