The most common cause is the intake air temperature sensor is not plugged in or it has stopped working. The second most likely cause are blockages in the exhaust, start by having the catalytics converters inspected. Other things to have checked are air filter dirtiness or blockages – we once saw a car have a filter fall out of a cheap OTR. Check fuel injectors for equal flow or have them cleaned.
This is often caused by a sick O2 sensor. The fuel trims build to maximum and then reset to zero and this is causing the rhythmic change that you have observed. Get them replaced.
The throttle position sensor is faulty and is not consistently reporting zero % during a no throttle condition. This is important because the engine enters idle mode based on this signal. Often the faulty throttle position sensor will report above 1% and this makes the car never enter idle mode and the engine rpm will vary up and down called hunting or will stay constantly high. This most often occurs when the throttle body area is hot. The way to fix it is replacement of the sensor and sometimes also involves modifying the mechanism if the connection point is worn.
Remote tuning allows tuning to be done remotely without the need to bring the vehicle to the workshop.
The sounds you are likely hearing are pinging which is caused by the fuel igniting too early in the engines cycle. The engine has excessive timing advance for the fuel that is in it at the moment. The engines tune calibration either needs to be changed or the fuel octane needs to be increased. Sometimes this can happen because someone has put 91 octane or E10 in a car that is tuned for 98.
Drive the car gently and keep the engine load away from where it pings until it is fixed. This can hurt your engine so take care. Other causes of pinging to start happening when it wasn’t before are blocked catalytic converters or blocked exhaust, dirty injectors which can happen slowly over time, failing fuel pump and more.
Cam upgrades require more air at idle than standard. The tune needs to be adjusted along with increased air being provided through the throttle body blade by a hole being drilled in the blade or the existing hole enlarged. Bigger lumpier cams require the largest holes. Take care when doing this because if the hole drilled is too big it may not be possible to have low enough rpm in coasting conditions without blocking up the hole and starting again.
The car needs to switch between coast spark advance table and drive spark advance tables. This switching is decided by throttle position. This needs to be fixed in the tune and this is assuming that the Air Fuel Ratios are good at low load.