Assuming your guitar isn't brand new, its guitar neck may have bowed (bent) under the pulling forces from the six strings. High humidity can swell up your guitar top. These two causes in combination can lead high string action.

If there are no other related issues, performing a set up job would help to rectify the problem. We would dehumidify your guitar, adjust your guitar truss rod, adjust your guitar nut or saddle if necessary, polish your fret wires and clean your guitar.

This gap may very small and no action is required yet. Here is a guide to help you in deciding if it can wait for not.

Use any dollar note and slip it between the gap and move the note around the edge of the bridge. Take notice of how deep the note is able to go in between the gap at difference spots. If you can do that easily plus at multiple spot around the bridge, it is time to bring it in for bridge re-gluing.

A dent is typically a small caved-in spot on your guitar. Firstly, check your guitar for its tuning stability and also play your guitar.

If your guitar plays well and sound right, the dent has no effects on the tonal quality of your guitar. You can leave the dent or you may want it repaired.

Repairing dents on guitar surface is classed as cosmetic repairs. Do know that it is time consuming and unbending in performing fine work regardless the size of the dent. The charging is proportional to the time and effort required.

Your guitar is unlikely a new piece but if it is, please bring it back to the seller to address the issue.

If a complete set up can't solve your guitar issue, it is probably your guitar's neck angle; the angle at which the neck is being set into your guitar body. Your guitar strings never stopped pulling. It can lead to compressions on your guitar neck joint (not immediately visible).

Even with slight compressions, it can alter your guitar neck angle that usually results in high string action yet saddle height remained low. A neck reset job may be required. It is a major job and it takes time and it wouldn't come at a low charge.

Without the luxury of inspecting your guitar, I could only make several educated guesses.

It is likely a localized issue. Since it occurs only with certain notes, it also means only when fretted it happens. Such buzzing could be due to uneven fret crown height, uneven fingerboard surface or worn-off fret wires. They can be resolved by leveling the "high" fret wires or if necessary, replacing the worn-off ones with new fret wires.

Certain strings also means buzzing occurs when open strings are played. In this case, there are multiple causes; (1) low saddle height, (2) low string height at nut, (3) backward bowed guitar neck, (4) certain fret wires protrude too much. The root cause(s) needs to be located first before performing any repairs.

There are a myriad of reasons to this. I shall not go into the electronics domain as I am not trained in it.

If your pickup system needs battery to operate, check if the battery has expired. A change of new batteries may help.

If your pickup system uses piezo undersaddle sensor, it is worth checking your saddle base for flatness. Piezo sensor takes the vibration energies from the strings and transform them into sound. A saddle with uneven base can reduce this energy transmission. Hence producing erratic outputs.

An acoustic guitar body is glued up of thin plys of wood, regardless it is full solid or not. Electric guitars are made of solid blocks of wood. Even electric guitars are less susceptible to it, both can suffer from cracks.

The crack can be on the finish surface or it could be a through crack. Both would require different approaches in their repairs.

Surface cracks are cracks on lacquer layers. Applying cosmetic repair methods can repair the cracked parts on lacquer layers.

Through cracks run through lacquer layers and wood plys. It requires both structural and functional repair methods. Firstly the crack areas on your guitar body need to be re-glued. Once the structure of your guitar is secured, cosmetic repairs can be applied to restore the lacquer layers.

Generally these cracks don't affect your guitar adversely. Unless your guitar top braces are detached due to these cracks, it may diminish your guitar's tonal quality. The acid test is still listening to your guitar carefully to decide if there are changes. In short, if the cracks don't hurt your guitar, you can decide not to bring it for repairs.

These repairs are not a stroll in the park. You should conside carefully before proceeding in them.

Rattling sound is made when multiple items are loosely hitting onto each other. If you hear such sound, it might be good to inspect the guitar's top and back braces for their structural integrity.

Get a mirror and torch, both should be small enough to enter the guitar soundhole. Manipulate them so that you are able to see all braces within the guitar body. Don't be surprised to see something unusual.

Regardless by air or ship, risks of transit damages to goods are very real. During transit, your guitar parcel would be subject to blunt forces. Your guitar can be smashed or squashed. Sometimes the braces may detach or they may crack under those stress.

All cracked braces have to be re-glued as soon as possible. Cracked braces are both structural and functional issues. The guitar's over strength is reduced and it won't play well.

Depending on the where these cracks are, each crack would require its suitable approach in glue application and clamping. The job is tedious and delicate. It may take days to complete.