If you drive your car quite often you will probably notice fairly quickly if it starts to sound different. It may be that the sound of the engine is louder or that when you change gear you notice a crunching sound. It may not be coming from the engine; it could instead be an issue with the suspension which may result in a knocking noise. It may be tempting to ignore this and wait until it gets worse but that is very risky, and you may end up having to pay a lot more to repair the issue later on down the line than if you had had it fixed when you first noticed it.
You will usually need to take it in to a garage to get a mechanic to have a look at it unless you are able to get a mobile mechanic out to you. Depending on what the noise is, they may ask you to leave the vehicle with them for a bit until they have had a chance to test drive it and investigate further.
If you want to be able to get quotes from other garages, be sure to make it clear that you do not want them to carry out any work until you have agreed for them to proceed.