Hi
If the crackling does not disappear within a few hours or comes back, then you may want to see a vet.
Any loss of appetite/weight and looking very off/ill will make it an emergency. In this case see a vet as soon as possible and step in with syringe feeding and watering support immediately. The need to breathe comes before the need to drink and only thirdly the need to eat - a piggy whose need to breathe is so bad that they stop eating and drinking is a very ill piggy indeed.
Guinea pig airways are very small narrow, and guinea pigs are not much in the way of mouth breathers, so even the least obstruction is very audible, so it is not at all easy to tell the difference.
Have you just opened a new bag of hay? Used perfumed/scented products etc. ?
See whether a bowl of steaming water may help to easy symptoms especially if yu are experiencing a cold spells and the heating has cranked up and made the indoors air very dry.
As with every health concern, please switch from the weekly weigh in and health check to weighing daily on your kitchen scales (cheap online or supermarket ones will do) first thing in the morning for best day to day comparison. Hay makes over three quarters of the daily food intake which you cannot control by eye so a piggy can lose a lot of weight without you noticing - the poo output is generally running 1-2 days behind the food intake.
As are you are based the US or Canada, URI is unfortunately much more likely compared to the UK. Sadly many of the shop bought piggies are infected with or have been exposed to the bacteria and could carry a reservoir that can come up again if the immune system is lowered for some reason.
Our forum is UK based but with we have members and enquiries from all over the world. It would help us a lot if you please added your country - nothing more if you wish - to location in your account details (click on your username on the top bar). This will allow us to automatically adjust any advice to conditions, differences when it comes to vet access, medical and other brands and climate etc. It helps both sides while still protecting your privacy.
Please take the time to read these links here; you may find them helpful since they contain all the practical how to and step by step advice, including how to improvise during an unforeseen emergency:
How Soon Should My Guinea Pig See A Vet? - A Quick Guide
Emergency and Crisis Care as well as Bridging Care until a Vet Appointment
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
Recommended exotics vets in the USA and Canada:
Guinea Lynx :: GL's Vet List
(These are the small animal specialists with experience with guinea pigs but they are also very expensive; with general vets you take your potluck as to whether they have experience with piggies or not since guinea pigs and other small pet rodents are classed 'exotics' and don't usually feature much on a general vet's curriculum. Vet fees are generally higher in North America anyway although the UK is fast catching up now, sadly).
All these links are part our of very useful new owners resource. You may want to bookmark this link, especially if you are in a different time zone:
Comprehensive Owners' Practical and Supportive Information Collection
All the best. Unfortunately, without access to your piggies, we have to always err on the side of caution. Regulations re. self-help and access to medication are also much stricter in the UK than the USA.