I know that when trying to determine if you have an AR(p) or MA(q) process, you look at the PACF and if it drops off significantly at a lag p, then you can say it's an AR(p), but if it's geometrically decaying and the ACF is significant till a lag, then it's likely an MA(q). But what if neither plot obviously shows these patterns?
For instance (looking at the attached pic), neither my ACF (of the twice differenced series) nor PACF (of once differenced & transformed series) are showing decay. Also neither shows a sudden drop off at a specific lag. If I had to guess I would propose an ARIMA(1,1,0), but again I'm really not sure.