What do your spark plug tips look like?
Engine knock can be caused by many things, however, some of the most common are:
The fuel in your car has too low of an octane rating -
The octane rating of gasoline is sometimes known as the AKI--anti-knock index. Manufacturers recommend a certain rating to maintain optimum performance in their vehicles. If you use gas that has an AKI that is too low, you can use an octane booster, purchased at any auto parts store, to bring it back to levels that will allow normal function. If you use the correct rating and it still has issues, try a different brand of gasoline. Using a gasoline with an AKI higher than the manufacturer recommends will not have any adverse affects. If you had just switched gasoline and your ECU hasn't yet adjusted to the new rating level, you can reset your ECU and bring your car's system back to default levels https://www.celicahobby.com/forums/ubbthreads.php/topics/267456/ecu-how-to-reset.html
Before doing this, if you have an ODB2 reader, plug it in and see what your engine is saying.
Carbon deposits on cylinder walls -
All fuels sold in the U.S. are required to have a certain level of carbon cleaning detergent in them, however this may not be enough to stop deposits from forming. When deposits form, the volume of the cylinder is effectively decreased, which increases compression that can cause knock. To combat this, first try to purchase some detergent additive at an auto parts store, and then try switching fuels. If your car is prone to knock, you may want to add the detergent at every fill-up. But first read through https://www.celicahobby.com/forums/...-engine-transmission-fuel-additives.html
Your Celica has incorrect spark plugs -
Manufacturers have recommended spark plugs for every vehicle, and often list some alternatives. If your car is running anything other than what is suggested, you may have knock. The heat range of a spark plug is often in its part number, and the spark plug is designed to run at that range. Its job--other than producing spark--is also to withdraw heat from the combustion chamber and into the head. Try to change your plugs if altering your fuel doesn't help.https://www.celicahobby.com/forums/ubbthreads.php/topics/369459/spark-plugs-info-how-to-replace.html
Do you also primarily drive in extreme cold weather? Do you let your Celica warm up before driving it, or do you just get in and go.